Hey gevelr,
i'm not certain if an NFS-Store would be the right choice for the Storage.
I know what you are trying to do. And i'm just warning you that you should think about "locking" :-)..
To move the Store i would go as followed..
And as further i look the more complicated it will become i guess.
If you really want to do this i would suggest you install a fresh system and configure everything prior to user-creation/migration.
On the other hand.. You could just mount /some/storage /var/lib/gromox/user :-).
This would obviously be the easiest solution.
But i would test the NFS-Store for at least a week with a current backup before you really do migrate!
Again, in a test-system.
cheers,
crpb