Redesigning a website, changing domain, or migrating hosting are decisions almost always made thinking only about the visual or technical side, and they can make organic traffic collapse if not planned carefully. Google has to "relearn" the new site from scratch, and that process can go well or very badly depending on how it's managed.
The costliest mistake: changing URLs without redirecting them
Every URL that changes without a 301 redirect to its new location becomes a 404 error for Google and for anyone who had that page saved or linked. All the ranking accumulated on that URL is lost instantly if it isn't redirected properly.
Before migrating: document everything that exists
The first step, before touching anything, is having a complete list of every indexed URL on the current site, its traffic and its rankings. Without that starting map, it's impossible to later know what was lost and where.
During the migration: a 1-to-1 redirect map
Every old URL should redirect to its closest equivalent on the new site, not all of them redirected in bulk to the homepage. A generic redirect to the homepage "because it's faster" is one of the decisions that costs the most traffic in poorly planned migrations.
After migrating: active monitoring for the first few weeks
Google Search Console should be checked closely in the weeks following the change: crawl errors, ranking drops, pages that stop being indexed. The sooner a problem is caught, the easier (and cheaper) it is to fix.
Frequently asked questions
Is it normal to lose some traffic during a migration?
A slight, temporary dip of days or a few weeks is normal while Google reprocesses the site. A large, sustained drop usually means something was done wrong (typically, missing or misconfigured redirects).
How long does it take Google to "adopt" the new site?
Anywhere from a few weeks to a few months, depending on the site's size and how often Google typically crawls it.
Do I need professional help to migrate without losing rankings?
For small, simple websites, it's feasible to do it carefully following a clear checklist. For large sites or ones with a lot of organic traffic, the financial risk of a mistake usually justifies getting specialised help.