Know what's working before you change it
Before anything is designed, export a list of the pages on your current site and which ones bring in traffic and enquiries. Google Search Console and Google Analytics will show you this.
Those pages, and the searches they rank for, are assets. The new site needs to keep or improve them.
Redirect every old address
If page addresses change, every old address needs a permanent (301) redirect to its closest new equivalent. Without them, links from other sites and existing rankings point at error pages.
This is the single most common cause of a post-launch drop, and it's entirely preventable with a simple redirect map.
Don't throw away good content
Pages that rank usually rank because of what they say. Rewriting them to be shorter or vaguer can lose that. Improve the content, but keep the substance that answers the searcher's question.
Get the technical basics right
Every page needs a unique title and description, one clear main heading and a sensible structure. The site should be fast on a phone, and the new pages should be in a sitemap submitted to Google Search Console.
Structured data that describes your business, services and location helps both Google and AI assistants understand the site.
Check tracking before launch day
Make sure Google Analytics, Google Ads conversion tracking and any call or form tracking are installed and tested on the new site before it goes live, so you can compare before and after.
Watch the first few weeks
Some movement in rankings after a launch is normal. Check Search Console for errors and pages that dropped out, and fix anything missed quickly. A well-planned redesign usually settles and then improves.