Site migration: find every old URL that will 404 before launch

Most migration traffic losses are plain 404s on old URLs that had links. Before you switch DNS:

1. Export the old URL list: old sitemap, plus analytics’ top landing pages, plus Search Console’s linked pages (sitemaps miss things).
2. Export the new sitemap from staging.
3. Diff the paths. Every old path without an identical new one needs a 301 to its closest equivalent, not to the homepage.
4. Write rules in your host’s format (Netlify _redirects, vercel.json redirects, nginx return 301, Cloudflare bulk redirects) and avoid chains.
5. After launch, request a sample of old URLs and confirm one hop to a 200.

Free: redirect-diff does step 3 from two sitemaps or URL lists. Redirects keep links working; they’re not a ranking guarantee.

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Disclosure: I build Routepatch, a $49 one-time pack that turns the gap into a host-ready redirect map. Its free check only offers the pack when it confirms the problem on your page. Everything above works without it.