How to redesign without losing the rankings you already have

Inventory, redirect mapping and the pages you should never quietly delete.

Inventory before design

Export every indexed URL and its traffic before a single wireframe is drawn. A redesign that cannot name its top fifty pages will delete some of them.

Redirect mapping is a deliverable

One-to-one redirects, not a blanket rule to the homepage. Then re-crawl after launch and fix what broke in the first week, not the first quarter.