changing the user id

[Jack Howarth]

     What is the easiest way to change the numerical value
of a user's user id under Fedora Core 2? I don't see the 
option to do that under the graphical Users and Groups
System Settings utility? Or do I just have to manually
edit /etc/passwd and /etc/passwd~ to change the user id
and then chown -R  on the user's home directory?
                        Jack
ps I need to do this to be able to exchange data off of
some SGI optical disks that all have a different user id
for the same user name.