I successfully and easily converted from mbox to Maildir format after finding 'perfect maildir' on the web.  Here is the URL... http://people.redhat.com/rkeech/#dovecot
 
There is a tar file that contains 3 perl scripts that should make it pretty easy for you.
 
Gary

 
On 11/9/05, Patrick <dovecot@puzzled.xs4all.nl> wrote:
Hi all,

New to the list. I have an updated FC4 box with Dovecot version
0.99.14.4.fc4. My mbox size is about 76 megs and growing rapidly because
of several high-volume mailing lists I'm subscribed to. I have noticed
that over time the performance of the Evoluion/Spamassassin/Dovecot
combo deteriorates. Someone on the Fedora users list suggested I have a
look at moving to a maildir setup. Reading the docs I can't really
figure out what to. Does anyone have a suggestion how I can safely move
my mbox setup to a maildir setup? Much appreciated!

Regards,
Patrick