Permanent redirects carry ranking signals to the new address; temporary ones do not. When a site is rebuilt, every old URL that had traffic or links needs a permanent redirect to its replacement, or the traffic is lost.
Redirects also have to agree with canonical tags. A site that redirects to www while declaring the apex as canonical is telling Google two different things, and Google resolves the contradiction by indexing neither.
A redirect map is written before a rebuild launches, with every old URL that had traffic or links in it, and it is tested on launch day, not after.