What is the most sensible or correct way to migrate ALL the users to the new system? Way in the past is was just perhaps copy the /etc/passwd file but I know thats not the case anymore. how do I easily recreate their account names etc... on the new machine.
When transitioning mail servers, I've always done this by writing a perl/PHP script to parse the passwd files and create "user only" passwd/shadow, group, gshadow, etc. files and then append those to the new system. For most recent RH based distros, users start at either 100 or 500, with the lower numbers reserved for system/daemon accounts. Then you can copy over the emails with rsync with the numeric IDs option set and it will all "just work".
My approach requires some programming knowledge; I don't understand why there isn't a standard set of scripts to use for transitioning mail servers; there may be.
Good luck!