UID conundrum

[jbk]

I have a backup server that runs backuppc and the backups are written to 
a set of usb disks that I rotate weekly. The Fedora is the distro. I 
have two partitions set aside so that I can rotate between the distro 
updates. Fedora 10 is currently running and I have installed F12 on the 
other partition.
Here is the issue. BackupPc as its current UID owns the files and backup 
data on the usb drives. The UID that was assigned to backuppc in F10 has 
been usurped by another program in F12. This is a problem because I 
either have to not test my install and just change the UID on all the 
backup disks to match the new assigned UID, which takes an hour or more 
per disk, or I have to change the UID for the conflicting programs by 
editing the /etc/passwd file and group file entries.
I have yet to boot into the F12 install.
So, is it possible to edit the UID's and not effect the ownership of key 
files if I have yet to boot into the system. When do UID's become 
effective?

Jim KR