Why rankings drop after a redesign
Google ranks individual pages. If a page that ranked is moved, renamed or deleted and nothing points to its replacement, that ranking goes with it. Pages whose text was shortened during the redesign also tend to slip.
Before you change anything
List every page that currently gets visits from Google. Search Console shows this under Performance. Also save a copy of the current titles, headings and page addresses. This list tells you which pages must survive.
Keep addresses, or redirect them
The simplest safe choice is to keep the same page addresses. If they must change, set a permanent (301) redirect from every old address to its closest new page. Redirecting everything to the home page does not work.
Keep the content that earns traffic
A cleaner design is not a reason to cut text on pages that rank. Keep the headings and the answers people searched for, and improve the layout around them.
After launch
Submit the sitemap again, check Search Console for crawl errors and watch the pages from your list for a few weeks. Small dips are normal. A page that disappears usually has a missing redirect.
Frequently asked questions
How long until rankings settle after a redesign?
It varies by site size and how much changed. Expect fluctuation for a few weeks and check the list of pages you saved before launch.
Should I change the domain name at the same time?
Preferably not. Changing the design and the domain together makes problems harder to trace. Do them separately if both are needed.