Website migration, explained
A website migration is any significant change that alters how search engines see a site: a redesign, a new content management system, a move to a new domain, a switch to HTTPS, merging sites, or restructuring URLs and navigation. Search engines have to rediscover and re-evaluate the site, and anything they relied on that disappears without a trace can take rankings with it.
The usual causes of lost visibility are predictable. Old URLs return errors instead of redirecting. Content that earned rankings is trimmed or merged away. New templates drop headings, internal links or structured data. Staging sites are indexed, or the live site launches with a noindex tag left in place. Each is preventable with planning.
A careful migration starts early: inventory every URL that earns traffic or links, map each one to its new home with permanent redirects, review templates before launch, test on staging, and then monitor crawling and indexing and rankings closely in Search Console. Some fluctuation after launch is normal; large, lasting losses usually trace back to a step that was skipped.