The highest-risk SEO operation.
BEFORE
Export a complete list of current URLs from analytics, Search Console and your sitemap Record current traffic and rankings for comparison Map every old URL to its new destination Take a full backup
DURING
Build and test the new site fully before switching Implement every redirect Update internal links to the new URLs Update canonical tags Regenerate the sitemap
AT LAUNCH
Check robots.txt is not blocking the site Check no noindex directives survived from staging Check the site is reachable and fast Submit the new sitemap Add the new property in Search Console if the domain changed, and use the change of address tool
AFTERWARDS
Monitor indexing and traffic weekly for a month Check 404 reports and fix missed redirects Expect a temporary fluctuation, with recovery over weeks
WHAT CAUSES LASTING DAMAGE
Missing redirects, redirect chains, a noindex left in place, robots.txt blocking, or redirecting everything to the homepage.