Ignoring old URLs, useful content and analytics can produce a better-looking website with weaker search visibility and fewer enquiries.
Audit before redesigning
Record current pages, traffic, rankings, links and conversions, then decide what should remain, merge or retire. Design should build on that evidence.
Protect URLs and measurement at launch
Retain useful URLs where possible and permanently redirect necessary changes. Recheck analytics, forms, sitemaps and error pages after release.