If you use imapsync to sync between the two servers, it's pretty painless - although the end users will have to rebuild their local mailboxes.
If you do what I did (598 gigabytes of email is just a WEE bit difficult to imapsync) and move the folders themselves, you'll have to get the courier-dovecot-migrate.pl script.
Look at my post (close to this one).
I upgraded the existing system to the latest version of ISPConfig, then created a new version on the new machine. The old was running courier, the new dovecot. I did a database dump of the dbispconfig, moved it to the new machine, then overwrote the dbispconfig database on the new box. I then ran the update.php from the same installer, told it dovecot, and let it rip. (This came directly from a post by Till). It still left three mentions of 'courier' in the system.
If you clear those 'courier' mentions with phpmyadmin, you can then move the /var/vmail/<domain>/user/* files into the /var/vmail/<domain>/user/Maildir/* file structure. (You'll have to make the Maildir if it doesn't show up automatically)
Once that's done, you'll run the migrate script like so.
./courier-dovecot-migrate.pl --to-dovecot --recursive --convert --overwrite /var/vmail/<domain>
(The script will automatically fix the file ownership issues that show up if you're using 'root' to create the Maildir)
Once you've done this, and it passes correctly (run it without the --convert first, as a test), _delete the courierimap* files from the folders_. This will probably take some scripting, or by hand work, but I found that Thunderbird, at least, was trying to read both the Dovecot subscriptions file AND the courier imap files, and ending up with phantom folders.
KEEP GOOD BACKUPS. Better yet, keep your old hard drive handy to plug into the new machine in case it does what happened to me, which was that updating the name of a couple of users caused the system to wipe the user email directories. Sixty-eight gigabytes then had to be recopied. (that was before removing 'courier' from the database. ) Yes, I had the old drive handy - I wasn't that stupid.