For converting to maildir I tried the suggestion listed here: http://www.howtoforge.com/forums/showthread.php?t=3
That allowed me to convert one user to maildir format. Unfortunately my other users don't have an mbox file in their home directories. God knows how or where they are storing mail. Fortunately these are just my personal email accounts and only the one that had the mbox file really gets much use. So everything looks good for my transition now.