[Dovecot] Moving from MBOX to Maildir or DBox ?

[Frank Bonnet]

Hello

Actually we use MBOX format for all our IMAP access but users complain 
they cannot create subfolders in imap folders, of course they cannot
because of MBOX format ...
I need gurus advices on changing mailbox format

Maildir of Dbox ?

What would be the safest way to go ?

Note : we extensively use Thunderbird here.

Thanks