[Exim] Smooth migration from Mailbox to Maildir?

Hi!

We had some problems recently. We changed our upstream provider but the
updates for the domain registry came to late. So for 48+ hours we could not
receive nor send mail because DNS entries were not updated in time.

After the updates we received about 1 GB of mail traffic for a total of 3 of
our users. The largest mailbox was 581MB. We currently use mailbox format.
Of course the user´s mail client timed out because the server was not able
to copy the mailbox file to the .pop file in time.

The idea is to switch to maildir format which appears to be more stable for
such cases. I hope that will work for such scenarios.

So the question is: How can I move smoothly from mailbox format to maildir
format? Do I have to use another pop server (currently we use popper wich
came with SuSE Linux 6.2)?

Exim Version is 3.14.

Any experience?

Thanks!

Daniel