It was I'm not sure who wrote the imap daemon; the pop3 daemon is vm-pop3d.
DirectAdmin themselves says you can't go backwards, so I'm not going to attempt it; you might want to contact their support department.
Switching to the right exim.conf file is the easy part, but you'll have to convert a lot of mailboxes backwards, and install the missing daemons.
Maybe a few other things as well, such as configurations.
Jeff