ah mail migration, one of the most irritating thing in the world.
Moving postfix files and /etc/passwd is NOT the way to do it.
The best and elegant way to do IMHO is to use imapsync (http://freshmeat.net/projects/imapsync/
). I assume you are using imap and this is just a great way to do it.
First you create all the mail accounts on the new centOS machine. Then you sync the emails from the old to the new. It will copy all the folders (draft, deleted, etc) and it will also have all the timestamps right. I just recently did this and as far as I know it is THE easiest way for me. You will need to know the username and password to do this sync.
You should install imapsync on another box FYI. I know installing the latest version of it required for me to add some perl modules and a newer version of perl. If you run into any snags let me know. I'll give you a hand. But I rather you try it on your own first and hit me up if the snags are too much.