Is your Interworx server saving emails in mbox or Maildir format?
If Interworx uses mbox format then you'll want to move all the mailbox files before you update DA to use Dovecot. If Interworx uses Maildir format, then you'll want to convert DA first.
If you're not sure where your files are stored or how to move them you might want to contact a system administrator with email experience to either consult with you or to do the email move.
Jeff