Replatforming is one of the few engineering decisions with an immediate, measurable commercial downside if handled carelessly. The new site is faster, the code is maintainable, and organic traffic falls forty percent because a few hundred URLs quietly stopped resolving.
Everything below is procedural. That is the point — the risk here is not technical difficulty, it is omission.
Inventory the old URLs before you touch anything
Export the existing sitemap, crawl the live site, and pull the pages that actually earn traffic from Search Console. Those three lists overlap but none of them is complete on its own. The union is the set of URLs that must resolve after the cutover.
Legacy content management systems accumulate URL shapes: dated post permalinks, custom post-type stubs, taxonomy archives, theme demo pages nobody remembers publishing. Each shape needs a decision — carry it forward, redirect it, or let it go deliberately.
Redirect once, to the right place
Every retired URL gets a single 301 to its closest equivalent. Not to the homepage — a homepage redirect is treated as a soft 404 and passes on nothing. If there is genuinely no equivalent, returning a 410 is more honest and resolves faster in the index.
Watch for chains. A redirect that lands on another redirect wastes crawl budget and dilutes the signal, and it is the usual consequence of changing trailing-slash behaviour at the same time as the URL structure. Decide the trailing-slash convention first and match the old one exactly where you can.
- One hop per redirect. Test the full list programmatically, not by sampling.
- Preserve canonical tags, structured data and metadata per page, not just sitewide.
- Keep the old install running read-only for a week or two after DNS moves.
Stage it, crawl it, then move DNS
Deploy to a staging subdomain with indexing disabled and crawl it end to end. Diff the resulting URL list against your inventory. Every entry should resolve with a 200 or a single 301 to a 200 — anything else is a finding to close before launch.
After the cutover, submit the new sitemap and watch the coverage report daily for a fortnight. Ranking movement in the first week is normal and usually settles. A rising 404 count is not, and the longer it runs the more expensive it becomes to unwind.
