[CentOS] dovecot/procmail w/maildir format?

If one has a large hierarchy of mbox files managed by dovecot and procmail, 
is there a convenient way to convert them to maildir?

Can one serve an account that uses both?

What about INBOX? Must that be a mbox file in /var/spool/mail/<user> or can 
it be a maildir directory in the user's home directory?

Can I change this per-user? I have some users with little organizational 
skill who build up huge inboxes and have only the default subfolders 
created by their mail client (ie. Sent, Trash, etc.), and I'd like to 
switch them to maildir to avoid the huge incremental backup load of backing 
up entire mail folders.