Thanks, I managed to write a script to individually convert this.
David, seriously, you could probably charge for such a script if it works dependably. If you can't share it here in the forums please PM me with a price.
I haven't had the occasion to try this (I'm still running mbox/UW-IMAP), but I understand that if you restore a cpanel backup of an mbox account, the restore process converts the mail from mbox to maildir if the server is running maildir/Courier. However, there are no reports that I can find of having this work the other way around.
Was chatting with someone yesterday who ran /scripts/convert2maildir and regretted it; then tried to go back to mbox with the same script (it appears to have that functionality as designed). However, as implemented, the script did a very poor job of backward conversion and left .cur and .tmp files and folders all over the place.
If cPanel can't get it right and you have, please share!