Website Migration SEO: The Complete Checklist (2026)
I’ve managed over 30 site migrations across e-commerce stores, SaaS platforms, publisher sites, and local business websites. Some went flawlessly. Others turned into months-long recovery projects that cost clients six figures in lost revenue. The difference between a smooth migration and a catastrophic one almost always comes down to preparation, not luck.
Website migration SEO is the process of preserving your search rankings, organic traffic, and link equity when making significant changes to your site’s structure, domain, platform, or design. Get it wrong, and you’re looking at months of traffic decline. Get it right, and you can actually improve your search performance.
Here’s the uncomfortable truth: according to an analysis of 892 migrations by Search Engine Journal, the average recovery time is 523 days. That’s nearly 18 months. And 17% of sites in that study never recovered their pre-migration traffic, even after 1,000 days. Those aren’t odds you want to gamble with.
This guide covers every step I use for client migrations, from the first planning meeting to the 90-day post-launch monitoring period. Bookmark it. Print it out. Tape it to your wall. Because when migration day arrives, you won’t have time to figure things out on the fly.
Why Site Migrations Are the Most Dangerous SEO Event
Algorithm updates get all the attention. Penalties make headlines. But in my experience, botched site migrations destroy more organic traffic than any Google update ever has.
Here’s why migrations are so risky:
- Everything changes at once. URLs shift, internal links break, page content moves, metadata gets overwritten, and structured data disappears. Google has to re-evaluate your entire site simultaneously.
- Redirect chains compound losses. Every redirect in a chain costs you roughly 10% of link equity. Two hops? You’ve lost 19%. Three? Nearly 27%.
- Crawl budgets get consumed. Googlebot has to process thousands of redirects, discover new URLs, and re-index everything. For large sites, this can take weeks or months. Understanding how crawling works is essential before attempting any migration.
- Mistakes compound silently. A broken redirect map doesn’t throw an error. A missing canonical tag doesn’t send an alert. You often won’t know something went wrong until traffic drops 3-4 weeks later.
I watched a large UK retailer lose approximately £3.8 million in first-month revenue during a £7.6 million site redesign. Their IT consultants overruled the SEO team’s redirect recommendations. The result? Thousands of high-authority pages returning 404 errors, months of recovery work, and a budget overrun that dwarfed the original project cost.
A separate study of 171 migrations found that 42% of sites never recovered their pre-migration traffic levels. That’s not a temporary dip. That’s permanent damage to the business.
The TransferWise-to-Wise domain migration tells the other side of the story. Their organic traffic initially dropped from 32.3 million to 13 million monthly visits after the rebrand. But because they executed the migration correctly, with comprehensive redirect mapping and preserved link equity, traffic eventually recovered to 205 million monthly visits. That’s a 500% improvement over the original domain. Proper migration SEO made that possible.
Types of Website Migrations
Not all migrations carry the same risk. The scope of change directly determines how much SEO disruption you’ll experience.
Domain Migration
Moving from one domain to another (e.g., transferwise.com to wise.com). This is the highest-risk migration type because you’re asking Google to transfer all trust signals, backlinks, and ranking authority to a completely new domain. Expected traffic drop: 40-70% with recovery taking 6-18 months.
Protocol Migration (HTTP to HTTPS)
Switching from HTTP to HTTPS. Lower risk than a domain change, but still requires 301 redirects from every HTTP URL to its HTTPS equivalent. I’ve seen sites lose 15-20% of traffic for weeks because they forgot to redirect HTTP versions of paginated URLs or parameter-based URLs.
Platform Migration
Changing your CMS (WordPress to Shopify, Magento to WooCommerce, custom to headless). URL structures almost always change, templates produce different HTML, and internal linking patterns shift. Expected traffic drop: 30-60% with recovery in 4-12 months.
Site Redesign
Updating the look and feel without changing platforms. This seems low-risk, but it can quietly destroy your SEO if navigation changes, content gets removed, or page templates alter heading structures. Expected traffic drop: 10-25% with recovery in 4-8 weeks.
Structural Migration
Reorganizing your URL hierarchy or site architecture without changing domains or platforms. Moving /blog/category/post-name/ to /post-name/ or consolidating subdomains into subdirectories. Every URL change needs a redirect. Every internal link needs updating.
Content Migration
Consolidating, pruning, or merging content. This includes combining thin pages, removing outdated posts, or merging duplicate content. While the risk is lower per-page, the cumulative effect of removing hundreds of indexed URLs can be significant if redirects aren’t handled properly.
Hybrid Migration
The most common (and most dangerous) scenario: combining multiple migration types at once. Rebranding + new platform + redesign + URL restructure. I strongly advise against this. If you must do it, budget 3x the timeline and 5x the QA effort.
Pre-Migration SEO Checklist (30+ Points)
Eighty percent of migration success is determined before you flip the switch. Rush this phase, and no amount of post-launch firefighting will save you.
Benchmarking and Documentation (Complete 30+ Days Before Migration)
- Export full crawl data from your current site. Use Screaming Frog, Sitebulb, or Ahrefs Site Audit. Capture every URL, status code, title tag, meta description, H1, canonical tag, and word count. This becomes your “source of truth” for the entire migration.
- Download Google Search Console data. Export Performance data (queries, pages, clicks, impressions, CTR, position) for at least the last 16 months. GSC only stores 16 months of data, and you’ll need this baseline to measure recovery.
- Export Google Analytics data. Pull page-level traffic, engagement metrics, conversion data, and revenue attribution for the last 12 months. Identify your top 100 pages by organic traffic.
- Document your current XML sitemap structure. Screenshot or download every sitemap file. Note which URLs are included and excluded.
- Catalog all backlinks. Export your full backlink profile from Ahrefs, Semrush, or Moz. Focus on pages with the most referring domains. These are your highest-priority redirect targets.
- List all indexed pages. Use the
site:operator and GSC’s Index Coverage report. Compare the number of indexed URLs to your crawl data. Discrepancies often reveal orphan pages or indexing issues that already exist. - Document structured data. List every page with schema markup, the type of schema, and whether it generates rich results. Schema frequently gets lost during migrations.
- Record current Core Web Vitals scores. Capture LCP, INP, and CLS for your top 50 pages. You’ll need these benchmarks to verify performance hasn’t degraded post-migration.
- Screenshot SERP features. If you hold featured snippets, People Also Ask inclusions, or image pack positions, document them. These are often the first things lost during a migration and the hardest to recover.
Redirect Planning (Complete 14+ Days Before Migration)
- Build a complete redirect map. Every old URL mapped to its new equivalent. No exceptions. Use a spreadsheet with columns: Old URL, New URL, Status Code (301), Priority (based on traffic/backlinks), and Verification Status.
- Prioritize redirects by value. Your top 100 traffic pages and top 100 backlinked pages get reviewed individually. Every other URL can use pattern-based redirect rules, but these high-value pages need manual verification.
- Handle pages with no equivalent. Some old URLs won’t have a direct match on the new site. Redirect them to the closest relevant page, not the homepage. Redirecting everything to the homepage is a soft 404 in Google’s eyes.
- Plan for parameter URLs. UTM parameters, session IDs, filter parameters, sort parameters. These generate thousands of URLs that need redirect rules. Regex-based redirects handle most of these, but test them thoroughly.
- Test redirects on staging. Map at least 200 redirect samples across different patterns. Manually verify they resolve correctly. Check for redirect chains (A → B → C should be A → C).
- Document redirect implementation method. Will you use .htaccess, nginx config, Cloudflare Page Rules, or server-side code? Each has different syntax, limitations, and performance implications.
Technical Preparation (Complete 7+ Days Before Migration)
- Verify robots.txt on the new site. The staging site’s robots.txt probably blocks all crawlers. Triple-check this gets updated before launch. I’ve seen multiple migrations where the staging robots.txt went live, blocking Google from crawling the entire site for days.
- Remove noindex tags from the new site. Staging environments use
noindex, nofollowmeta tags to prevent accidental indexing. If these transfer to production, your entire site disappears from search results. Verify by viewing page source on the new site. - Verify canonical tags on the new site. Every page should have a self-referencing canonical pointing to the new URL, not the old one, and definitely not to the staging domain.
- Configure DNS TTL values. Lower your TTL to 300 seconds (5 minutes) at least 48 hours before migration. This ensures DNS changes propagate quickly. After migration stabilizes, set it back to 3600 or higher.
- Set up server-side monitoring. Configure uptime monitoring, error rate alerts, and server response time tracking. You need to know immediately if the new site goes down or starts throwing 500 errors.
- Prepare a rollback plan. Document exactly how to revert to the old site if the migration fails catastrophically. This includes DNS settings, database backups, file system snapshots, and a clear decision tree for when to pull the trigger.
- Run a full SEO audit on the staging site. Crawl the new site before launch. Check for broken internal links, missing metadata, duplicate content, orphan pages, and accessibility issues. Fix everything before go-live.
- Test mobile rendering. Google uses mobile-first indexing. If the new site’s mobile experience is broken or degraded, rankings will suffer regardless of how good the desktop version looks.
- Verify hreflang tags (international sites). If you serve multiple languages or regions, confirm hreflang annotations are correct on the new site. Getting these wrong can send French users to English pages and vice versa.
- Check internal linking. Run a crawl comparison between old and new sites. Count internal links per page. If key pages lose significant internal links during the redesign, their rankings will drop even with perfect redirects.
Stakeholder and Communication Prep
- Set realistic expectations. Tell stakeholders: “Traffic will dip. Rankings will fluctuate. This is normal and expected.” Nobody can predict exact migration impact, and pretending otherwise sets you up for blame when the inevitable dip occurs.
- Define success metrics. What does a successful migration look like? I typically target 90-95% traffic recovery within 30 days and full recovery within 90 days. Document these targets so everyone’s aligned.
- Schedule the migration window. Avoid peak traffic periods, major product launches, or holiday seasons. Launch during off-peak hours (early morning or late evening for your primary traffic region).
- Assemble the migration team. You need: SEO lead, backend developer, frontend developer, QA tester, project manager, and someone with server/DNS access. Everyone should be available for the full 48 hours post-launch.
- Create a communication plan. Who gets notified when the migration starts? Who monitors what? What are the escalation paths? Who has authority to trigger a rollback?
- Prepare Google Search Console. If changing domains, set up the new domain property in GSC before migration. Verify ownership. You’ll need both the old and new properties active for the Change of Address tool.
- Back up everything. Full database backup, complete file system backup, theme/template backup, plugin/module list, and server configuration backup. Store backups off-server. Test that backups can actually be restored.
During Migration: The Critical 48 Hours
Migration day isn’t when the real work happens. It’s when you verify that all the preparation works as planned. But these 48 hours determine whether your next quarter is spent growing or recovering.
Launch Day (Hours 0-24)
- Deploy the new site to production. Follow your deployment plan exactly. No last-minute changes. No “quick fixes” that weren’t tested on staging.
- Verify redirects are active. Test your priority redirect samples immediately. Check that 301s are returning correctly, not 302s. Use curl or a bulk redirect checker tool for speed:
curl -I -L https://oldsite.com/important-page/ - Confirm robots.txt allows crawling. Visit yourdomain.com/robots.txt and verify it’s the production version, not the staging blocker. Test in Google’s robots.txt tester.
- Verify noindex tags are removed. Spot-check 20+ pages across different templates. View source, search for “noindex.” One missed template type can deindex an entire content section.
- Submit updated XML sitemaps to Google Search Console. Remove old sitemaps, submit new ones. If you changed domains, submit sitemaps to both old and new GSC properties.
- Use GSC’s Change of Address tool (domain migrations only). This tells Google explicitly that your site has moved. It accelerates the transfer of ranking signals to the new domain.
- Monitor server logs in real time. Watch for 500 errors, spikes in 404s, redirect loops, and unusual response times. The first hour of crawling activity reveals most critical issues.
- Verify analytics tracking. Check that GA4, Google Tag Manager, and any conversion tracking are firing correctly on the new site. Missing analytics doesn’t affect SEO directly, but it blinds you to problems.
- Test Core Web Vitals on live pages. Run PageSpeed Insights on your top 10 pages. Compare to pre-migration benchmarks. A significant LCP or CLS regression needs immediate attention.
- Check structured data. Use Google’s Rich Results Test on key page types (homepage, product, article, FAQ). Verify schema is rendering correctly with no errors.
First 48 Hours
- Monitor crawl activity in GSC. Check the Crawl Stats report. Are Google’s bots finding your new URLs? Are they encountering errors? A spike in “Not found (404)” responses means your redirect map has holes.
- Check index coverage daily. Watch for increases in “Excluded” pages, “Crawled – currently not indexed,” or “Discovered – currently not indexed” statuses. These flags indicate Google is struggling to process your migration.
- Spot-check high-priority pages. Search Google for
site:yourdomain.com/important-pageon your top 20 pages. Verify the correct (new) URLs are appearing in search results, not old URLs or duplicates. - Monitor 404 errors. Check server logs and GSC for pages returning 404 that should be redirecting. Fix immediately. Every day a high-traffic page returns a 404 costs you accumulated ranking signals that are incredibly hard to recover.
- Verify internal links. Run a quick crawl of the live site. Check that internal links point to new URLs directly, not through redirect chains. Updating internal links to point to final destinations reduces crawl waste and preserves link equity.
- Test across devices and browsers. Check mobile, tablet, and desktop rendering. Test Chrome, Safari, Firefox, and Edge. Rendering issues can cause Googlebot to see different content than users, triggering ranking drops.
- Watch for redirect chains. Any URL that redirects more than once is a chain. Old URL → intermediate URL → new URL wastes crawl budget and leaks link equity. Flatten all chains to single 301 hops.
Post-Migration Verification Checklist
The migration isn’t “done” when the new site goes live. The real verification happens over the next 90 days.
Week 1 (Days 1-7)
- Daily GSC monitoring. Check Performance, Index Coverage, and Crawl Stats reports every day. Look for sudden drops in impressions (often the first signal of a problem).
- Verify indexing of priority pages. Use the URL Inspection tool in GSC for your top 50 pages. Request indexing for any that haven’t been crawled yet.
- Monitor server response codes. Run a weekly crawl and compare status codes to your redirect map. Every 404 on a previously-ranked URL is lost traffic.
- Check backlink landing pages. Verify that your top 100 backlinked URLs correctly redirect to the new equivalents. Broken backlink destinations mean lost link equity.
- Test all conversion paths. Forms, checkout flows, lead gen funnels, and phone number click-to-call. Revenue impact from broken conversions is more immediately painful than traffic loss.
Weeks 2-4 (Days 8-30)
- Compare traffic to pre-migration baseline. Use the benchmarks you captured before migration. A 10-15% traffic dip during the first 2-4 weeks is normal. A 30%+ drop signals a problem that needs investigation.
- Monitor keyword rankings. Track your top 100 keywords daily. Some volatility is expected, but if rankings for core terms keep declining past week 3, investigate the affected pages specifically.
- Audit internal links on the new site. Run a full crawl and generate an internal linking report. Identify any pages that lost significant internal links during the migration. Add links back manually.
- Check for soft 404s. Pages that load but return thin or irrelevant content (often caused by bad redirects) show as soft 404s in GSC. Fix these by updating redirects to more relevant destination pages.
- Review SERP feature retention. Check whether you’ve maintained featured snippets, People Also Ask appearances, and other SERP features. Compare to your pre-migration screenshots.
- Update external references. Reach out to sites linking to you with the old URLs (particularly for domain migrations). While 301s pass equity, direct links to the new URLs are cleaner.
Months 2-3 (Days 31-90)
- Measure against success metrics. Compare current traffic to your 90-95% recovery target. If you’re falling short, run a comprehensive technical SEO audit to identify remaining issues.
- Analyze page-level performance. Don’t just look at sitewide traffic. Some pages may have recovered while others are still struggling. Investigate underperforming pages individually.
- Monitor for ranking stability. By month 3, rankings should stabilize. If you’re still seeing significant weekly volatility, there are likely unresolved technical issues (redirect chains, canonical conflicts, or crawl errors).
- Clean up redirect chains. Some chains only become apparent after Google has fully processed the migration. Run a redirect audit at the 60-day mark and flatten any chains that developed.
- Document lessons learned. What went well? What went wrong? What would you do differently? This documentation is invaluable for future migrations.
How Long Until Rankings Recover?
The honest answer? It depends entirely on the migration type, your site’s authority, and how well you executed the checklist above.
Here’s what the data shows across different migration scenarios:
| Migration Type | Typical Traffic Drop | Well-Executed Recovery | Poorly-Executed Recovery |
|---|---|---|---|
| Site Redesign (same URLs) | 10-25% | 4-8 weeks | 3-6 months |
| Platform Migration | 30-60% | 4-12 weeks | 6-12 months |
| Domain Migration | 40-70% | 6-18 months | Never (17% of cases) |
| Protocol (HTTP to HTTPS) | 5-15% | 2-4 weeks | 2-3 months |
| Structural Reorganization | 15-35% | 6-12 weeks | 4-8 months |
A study of 892 migrations found the fastest recovery took just 19 days with exceptional planning and execution. The same study found 17% of sites never recovered, even after 1,000+ days.
In my experience, well-executed migrations follow a predictable pattern:
- Days 1-7: Impressions drop as Google recrawls and reprocesses. Rankings fluctuate. This is normal.
- Days 7-21: Google begins associating the new URLs with your previous ranking signals. You’ll see partial recovery.
- Days 21-60: Most rankings stabilize. Pages with strong backlink profiles recover fastest.
- Days 60-90: Full recovery for well-executed migrations. Pages still underperforming at this point likely have specific technical issues.
One factor that doesn’t get discussed enough: domain authority matters for recovery speed. High-authority sites (DA 60+) recover faster because Google already trusts those domains. A DA 15 site migrating to a new domain faces a much longer recovery because there’s less accumulated trust to transfer.
Common Migration Mistakes That Destroy Traffic
After managing dozens of migrations, I’ve seen the same mistakes repeatedly. Every one of these has cost a real business real money.
1. Redirecting Everything to the Homepage
When the redirect map gets complicated, some developers take a shortcut: redirect every old URL to the homepage. Google treats this as a soft 404. You lose every ranking those pages held. I’ve seen this single mistake wipe out 80% of a site’s organic traffic overnight.
2. Using 302 Redirects Instead of 301s
A 302 says “this page has temporarily moved.” A 301 says “this page has permanently moved.” Google handles them differently. A 302 doesn’t pass full link equity because Google expects the original URL to come back. During a permanent migration, every redirect must be a 301.
3. Leaving Staging Site Noindex Tags Live
The staging environment should block search engines with noindex and robots.txt. But if those directives transfer to the production site, you’ve just told Google to deindex your entire website. I’ve seen this happen twice in my career. Both times it took 3-4 weeks to fully recover indexing.
4. Not Updating Internal Links
Redirects handle external traffic and backlinks, but internal links should point directly to the new URLs. Every internal link going through a redirect is wasted crawl budget and diluted link equity. On a 10,000-page site, this compounds into a massive crawling inefficiency.
5. Forgetting About Images
Images carry SEO value through alt text, file names, and Google Images rankings. If image URLs change during a migration and aren’t redirected, you lose all that value. I’ve seen e-commerce sites lose 30% of their Google Images traffic after a platform migration because image redirects were never implemented.
6. Changing URLs and Content Simultaneously
If you change the URL and significantly alter the content at the same time, Google has to re-evaluate both the location and the relevance of the page. It’s much harder to transfer ranking signals when the destination page looks nothing like the original. Migrate first, then optimize content separately.
7. Ignoring Pagination and Filtered URLs
Category pages with pagination (/category/?page=2, /category/?page=3) and filtered URLs (/category/?color=red&size=large) often get overlooked. These URLs have been indexed, they have link equity, and they need redirect rules.
8. No Monitoring After Launch
Launching the new site and walking away is shockingly common. The migration team celebrates, disbands, and nobody checks GSC for two weeks. By then, thousands of 404 errors have accumulated, redirect chains have formed, and ranking recovery has been significantly delayed.
9. Migrating During Peak Season
An e-commerce client once insisted on migrating their platform in November, three weeks before Black Friday. Traffic dipped 35% during what should have been their highest-revenue month. The recovery didn’t complete until February. That’s three months of peak season revenue compromised by bad timing.
10. Not Having a Rollback Plan
If the migration fails catastrophically (and some do), you need the ability to revert within hours, not days. Without tested database backups, documented DNS settings, and a clear rollback decision tree, you’re trapped on a broken site with no escape route.
When NOT to Migrate
Sometimes the best migration advice is: don’t do it. Not every site needs to move, and the risks often outweigh the benefits.
Don’t migrate if your primary motivation is visual. A new design doesn’t require a platform change. Most CMS platforms support complete visual redesigns without changing a single URL. If all you want is a fresh look, hire a designer, not a migration team.
Don’t migrate during a traffic upswing. If your site is currently growing in organic traffic, a migration will interrupt that momentum. Wait until traffic plateaus, then migrate from a position of stability.
Don’t migrate without dedicated SEO resources. If nobody on the project team understands redirects, crawl budgets, canonical tags, or index management, the migration will fail. Budget for SEO involvement from planning through the 90-day monitoring period.
Don’t migrate close to peak business periods. Give yourself at least 3 months of buffer between the migration and your peak season. E-commerce sites should avoid migrating between September and January. B2B SaaS companies should avoid migrating during their renewal period.
Don’t combine multiple migration types. Rebranding + new platform + new URL structure + new content = a recipe for disaster. If you must make multiple changes, phase them. Change platforms first, stabilize for 3 months, then rebrand. Each phase gives Google time to process the changes.
Don’t migrate a site with existing SEO problems. Fix indexing issues, crawl errors, duplicate content, and broken redirects before migration. Migrating a broken site just transfers the problems (and often creates new ones).
Don’t migrate because a salesperson told you to. Platform vendors, agency new-business teams, and design firms all have financial incentives to recommend migrations. Evaluate the business case independently. The SEO risk of migration is real. The promised benefits of the new platform may not be.
FAQ
How much traffic will I lose during a website migration?
A well-executed migration typically sees a 10-25% traffic dip that recovers within 4-8 weeks. Poorly executed migrations can lose 50%+ and take 6-18 months to recover. Domain migrations carry the highest risk, with traffic drops of 40-70% being common even when done correctly. The key variable is redirect accuracy: if 95%+ of your redirects are correct and pointing to relevant content, your recovery window shrinks dramatically.
Do 301 redirects pass all link equity?
Google has confirmed that 301 redirects pass full PageRank (link equity) to the destination URL. However, redirect chains (multiple hops) compound losses. Each additional hop in a redirect chain can cost approximately 10% of link equity. This is why flattening redirect chains to single hops is critical during migration cleanup.
Should I keep the old domain active after migrating?
Yes. Keep the old domain registered and actively redirecting for at least 12-24 months after migration. Google needs time to fully process the domain change, and backlinks pointing to the old domain need to continue resolving. Some SEOs recommend keeping old domain redirects active indefinitely, which costs very little compared to losing accumulated link equity.
How do I handle pages that no longer exist on the new site?
Redirect them to the most relevant existing page on the new site. If a product page no longer exists, redirect to the parent category. If a blog post topic was merged with another, redirect to the merged post. Never redirect removed pages to the homepage. Google interprets mass homepage redirects as soft 404s. If there’s genuinely no relevant destination, a 410 (Gone) status code is more appropriate than a misleading redirect.
Can a migration improve my SEO?
Absolutely. The TransferWise-to-Wise migration eventually resulted in a 500% traffic increase over the original domain. Migrations that improve site speed, fix longstanding technical debt, upgrade to better URL structures, or consolidate domain authority from multiple properties can produce significant long-term SEO gains. But the short-term dip is almost always unavoidable, even in the best-case scenario. Only about 10% of migrations actively improve SEO performance.
What’s the difference between a 301 and a 308 redirect?
A 301 is a permanent redirect that may change the HTTP method (POST can become GET). A 308 is a permanent redirect that preserves the HTTP method. For SEO purposes, 301 is the standard and preferred redirect for migrations. Google processes 301 and 308 similarly for ranking signal transfer, but 301 has wider browser and crawler support. Stick with 301 unless you have a specific technical reason to use 308.
How soon should I submit sitemaps after migration?
Submit updated XML sitemaps to Google Search Console within the first hour after the new site goes live. Remove references to old sitemap files and submit new ones. For domain migrations, submit sitemaps to both the old and new GSC properties. This accelerates Google’s discovery of your new URL structure and speeds up the re-indexing process.
Should I use Google’s Change of Address tool?
For domain migrations, yes. Google’s Change of Address tool (in Search Console under Settings) explicitly tells Google that your site has moved to a new domain. It helps Google transfer ranking signals and accelerates the migration process. This tool is only for domain changes, not for URL restructuring within the same domain.
My traffic hasn’t recovered after 90 days. What should I do?
Run a comprehensive SEO audit on the new site. Common post-migration issues that prevent recovery include: redirect chains that developed after launch, canonical tag conflicts between old and new URLs, missing or incorrect hreflang tags (for international sites), pages excluded from indexing due to noindex tags or robots.txt rules, and significant internal link loss. Check GSC’s Index Coverage report for clues, and crawl the site to compare its current state to your pre-migration benchmark data.