Save a map of the current site
Collect the URLs, headings, metadata, main content and whatever traffic data you have. This will later become the basis for SEO on the new site. Mark your priority pages and the content you plan to merge. Do not judge a URL's value by how it looks in the old menu.
For each old page, make a decision: keep the URL, redirect it to the matching new page, or remove it with no equivalent replacement. Redirecting everything to the homepage does not tell the visitor where to find the answer they need.
Redirect to equivalent content
Prepare a table: old URL → new URL. Check that each target loads and still meets the original need. Avoid unnecessary redirect chains, where a URL goes through several hops before it lands.
Google's guidance on site moves with URL changes covers URL mapping, redirects and monitoring after the move. Even with careful preparation, changes need watching; promising zero fluctuation would be misleading.
Check the result after launch
Crawl the old and new URLs again, check links, canonical URLs (new website acceptance) and any accidental blocks on indexing. A staging noindex tag must not stay on the live site by mistake, and the staging copy should not be opened up to search engines.
Before a website rebuild, it is worth running an SEO audit and including the URL migration map in the specification. After launch, track errors and impressions for specific pages, not just the overall traffic number.
