[exim] Concurrent use of mbox and maildir formats

Hi,
I'm in the process of switching over from ipop3d to courier-imap, and as
part of that, switching my users from mbox to maildir.

This is quite a big job, and isn't practical to do in one evening, so I need
to migrate the users over several days, while still allowing them to get
access to their emails.

I've set up a test server with exim and courier-imap, and I'm in the process
of trying this out, prior to doing it on the live server.

I've managed to set up the necessary routers in the exim config, and I can
have some test user accounts using their existing mbox files, while others
are using the new maildir files.

However the problem comes when retrieving mail. As courier-imap (i assume)
cannot look at mbox files, any of my test accounts still using mbox files,
cannot retrieve their emails, while the ones using the new maildir files
can.

One obvious solution would be to run a pop3 server than can read mbox files,
concurrently with courier-imaps pop3 server and have them listening on
different ports - then setting the mail clients to the appropriate port for
POP3.

Has anyone else had to do this this kind of migration ? Anyone have any
suggestions ? I've tried to install ipop3d, but it won't install with
courier-imap (conflicting files).

Mike