[Dovecot] Want to have some users with Maildir, some with mbox

[WJCarpenter]

> BTW. Are you also using a script that preserves IMAP UIDs? Otherwise 
> IMAP clients will re-download all the mail.

Does the "convert" plugin preserve the UIDs?  The wiki implies that it 
doesn't, but I thought maybe since it's implemented as a plugin that 
maybe it does (or that maybe the caution is about converting to/from 
dovecot and some other server).