[Blind-sysadmins] migrating imap messages to new server

Hello, I've got two imap servers one old, one new. The old one has messages on it that i'd like to get on to the new one for several users. Both servers are using open source software, postfix for the MTA and Dovecot for the imap server. If anyone knows how to pull this off I'd like to know about it. I would try copying them,but doubt it's that simple, I want to preserve their dates, times, and attributes. Thanks. Dave.