[Dovecot] Dovecot 2.0.3

I upgraded today to from 1.2.12 to Dovecot 2.0.3 with the dbox format. I just figured out there are still mail in some users directories with the maildir format. These mail are not displayed at all. Nether with roundcube nor with Thunderbird. How could I convert the remaining maildir mails to dbox?

Thanks Matthias