How to change some GID

[Olivier Vogel]

Let me first explain my situation.

I would like to introduce Ubuntu in my school. All the computers are
Macintosh, working with Mac OS X. There is a ldap database of users and
all the home directories are on a server (with Mac OS X Server). I did
success to manage the login to the server. So the problem is not here.

The problem begins with the authorizations of the users. I have
understand that the users must belong to the groups:
gid 24 for cdrom (access to cdrom)
gid 25 for floppy (access to floppy)
gid 29 for audio (have sound and listen music)
gid 46 for plugdev (can mount usb stick and so on) 
gid 108 for scanner (can use a scanner)

BUT, the administrator of Mac OS X Server is unable to create such
groups with the same gid on the ldap database. All the groups that he
can create have a gid > 500.

So, I have tried to change the gid of these groups:
cdrom -> 2024
floppy -> 2025
audio -> 2029
plugdev -> 2046
scanner -> 2108

Only the change for the audio group has succeeded and not the others. I
have tried with Terminal
sudo groupmod -g 2024 cdrom (without success)
sudo groupmod -o -g 2024 cdrom (seems to success)

But, after these changes, the automounting of cdrom doesn't work
anymore. So I have to restore cdrom to gid 24.

Please help me to solve this problem; otherwise my school will never
know Ubuntu!!!

Thanks for your help.

Olivier