Install F-whatever. Create the first user as install
and let F allocate the default UID /GID to that user.
If the install user's default UID clashes with one of your current user's UIDs then reinstall but use that (current user's) name and PW for the default "first user" at the install. Then proceed.
Now create usernames for all your other users, but make absolutely sure that usernames and groupnames match the UIDs / GIDs from your old install. Otherwise life becomes very
Additionally, what if the fresh install creates user ids for system services that bump into older ones?
You will have to add the users yourself and make sure this does not happen.