September 23, 2026
How to Fix 404 Errors After a Website Migration: URL Mapping Guide
A practical migration workflow for inventorying legacy URLs, mapping old paths to relevant new pages, testing redirects, and monitoring missed 404 traffic after launch.
A redesign, CMS move, domain change, or URL-structure migration can launch successfully while still losing traffic through old URLs. Search engines, backlinks, ads, email campaigns, and bookmarks may continue requesting paths from the previous site.
Build a legacy URL inventory before launch
Collect URLs from multiple sources: XML sitemaps, Search Console, analytics landing pages, server logs, backlink data, CMS exports, active ads, and email campaigns.
Do not rely on the sitemap alone. Valuable historical URLs may no longer appear there.
Build an old-to-new URL map
Each old URL should have the most relevant destination possible.
old product → same product on new site
old category → equivalent category
old guide → updated version
no meaningful replacement → evaluate 404/410
A useful mapping sheet can include old URL, new URL, redirect type, reason, priority, and test status.
Do not map everything to the homepage
Bulk homepage redirects destroy the relationship between the original intent and the destination. If no relevant replacement exists, an accurate missing-page response may be better.
Test before launch
For every important mapping, verify:
- expected redirect status
- correct final destination
- final page returns 200
- no redirect chain
- no redirect loop
- protocol/domain normalization does not create unnecessary hops
Monitor after launch
Even a strong mapping plan will miss some URLs. Forgotten backlinks, old campaigns, legacy feeds, and historical structures often appear only after real traffic arrives.
Post-launch 404 monitoring is therefore part of the migration—not an optional cleanup step.
How no404 fits in
no404 can surface real 404 requests that the migration map missed.
Example:
/legacy-category/product-x → not in map → real visitor hits 404 → request becomes visible → /products/product-x identified → redirect added.
This creates a feedback loop where the migration improves based on real traffic.
Metrics worth tracking
Track unique 404 URLs, human 404 visits, recovered visits, organic landing-page drops, 404 URLs with backlinks, and paid/UTM traffic reaching missing destinations.
Conclusion
Successful migrations require two layers: a planned redirect map before launch and live 404 monitoring afterward. no404 supports the second layer by revealing important paths that static migration planning missed.