You can also migrate existing mbox folders to maildir if need be by
installing mb2md which will set everything up in the correct format and
also parse and relocate existing mbox folders. I've done this before for
dovecot with mbox folders with over a year of mail with no problems. It
will also create the cur, new, tmp stuff for you.
Dovecot does a nice job of scanning home directories and locating
supported mailboxes and I've used it with fetchmail, spamassassin,
postfix, procmail for quite awhile now.