The most common way businesses lose rankings.
BEFORE THE REDESIGN
Export all current URLs from analytics, Search Console and your sitemap Record current traffic, rankings and top pages Identify which pages bring traffic and conversions
That list is what you must preserve.
DURING
Keep URLs unchanged wherever possible. Every changed URL is a redirect to manage and a risk. If URLs must change, map every old one to its new destination. Preserve the content on pages that rank. A redesign that halves the text on a ranking page usually halves its ranking. Keep title tags and descriptions, or improve them deliberately rather than losing them.
AT LAUNCH
Implement every redirect, in one hop Check robots.txt is not blocking Check no noindex survived from staging Regenerate and submit the sitemap Update internal links
AFTERWARDS
Monitor weekly for a month. Check 404 reports for missed redirects.
THE COMMON DISASTER
A beautiful new site with new URLs, no redirects, and a noindex left from development. Traffic disappears within weeks.