Both, Nvidia and AMD have options for full, clean driver installation/upgrade with or without keeping settings. DDU is most usable when changing one to another make of GPU and Sound so there are no unnecessary files left. There's usually no harm in using DDU though, only "danger" is that Windows might not recognize your new GPU right away and leave you without driver and therefore a picture at all because DDU is done in Windows safe mode,
Replacing Windows is grossly overkill, it would take me over a week to get them to same state as before and would accomplish nothing unless Windows are screwed up anyway.