A site migration is any significant change to a website’s domain, platform, URL structure, protocol or hosting that alters how search engines find, crawl and rank its pages. Moving from a .co.uk to a .com, switching from Wix to WordPress or reorganising every service page under a new folder all count.
How a site migration works
Search engines know your site as a list of URLs, each with its own history: links pointing at it, the queries it ranks for and the signals Google has built up over time. A migration changes some or all of those addresses, or the code that sits behind them. The job is to hand that history over to the new version without dropping any of it.
The main types are:
- Domain change Such as a rebrand or merging two businesses onto one domain.
- Platform change (replatforming), such as Magento to Shopify. URLs often change because each platform has its own patterns.
- Structure change Where pages move into new folders or are merged and split.
- Protocol or hosting change Such as HTTP to HTTPS or a move to a new server.
The mechanism that carries value across is the 301 redirect: every old URL permanently forwards to its closest new equivalent. Around that sit the supporting signals, including updated internal links, canonical tags, an XML sitemap of the new URLs and, for a domain change, the Change of Address tool in Google Search Console.
Why it matters
Of all the things that can go wrong in SEO, a badly handled migration does the most damage in the shortest time. A UK business can lose most of its organic enquiries in the weeks after a new website goes live, not because the new site is worse, but because nobody mapped the old URLs. Every blog post with links from local press, every service page that ranked well, suddenly returned a 404.
Migrations also tend to happen at the worst moment: alongside a rebrand, a new owner or a busy season. Planning the SEO side early costs far less than recovering lost visibility afterwards.
Common mistakes
- No redirect map. Redirecting everything to the homepage, or relying on the developer to catch the important pages, loses the value of the rest.
- Launching with the staging block still on. A noindex tag or a robots.txt disallow left over from the staging site can remove a whole site from Google within days.
- Redirect chains. Old redirects from previous redesigns pointing at URLs that now redirect again.
- Losing content. Pages that ranked are cut or shortened because they did not fit the new design.
- Broken tracking. GA4 tags or conversion events missing on the new templates, so nobody can tell what has happened.
- Changing everything at once. A new domain, platform, design and content in a single launch makes it impossible to work out what caused a drop.
How to act on it
Start with a benchmark: export traffic by landing page from GA4, performance by page from Search Console, and a full crawl of the current site. List every URL with traffic, impressions or links from other websites. Map each one to its new address, then build the rules. The redirect map builder turns an old-to-new list into rules and flags chains and loops.
Before launch, crawl the staging site and check titles, headings, canonicals and structured data against the old site. On launch day, remove any staging blocks, crawl the old URL list to confirm each returns a single 301 to a live page, and submit the new sitemap. Then watch Search Console daily for the first fortnight. The website launch SEO checklist covers each step, and if you would rather hand the job over, this is the work I do in website migration SEO.