Saved for future reference, since I don’t do it very often…

1. From another admin account, open System Preferences, click Accounts.
2. Control-click the username, select Advanced Options.
3. Change the UID and optionally the GID.
4. From a root shell, chown -R the user's files.
5. If you changed the GID, run:
   dseditgroup -o edit -a $USER -t user $OLDGROUP
6. If you want to add more local groups to match your NFS server, run:
   dseditgroup -o create -r $DESC -i $GID $GROUP