Migrating from 10.3.9 server to 10.4.10

Hello My server is currently running 10.3.9 and hopefully soon I'll be mailing a new hard drive to the data center that has 10.4.10 Server on it. All the web and database, vpn etc stuff is easy. Its just going to take time but otherwise will be a piece of cake to set up. However the mail has me a little confused. I have about 15 email accounts running on the server. All are users made in Workgroup Manger. We're talking about a GB or so of very important email. All IMAP. So what is the best procedure to use, to go about getting the users and mailboxes to the new drive, which will essentially be a new machine?