We have decided to migrate one of our mail servers to use Dovecot instead of Cyrus IMAP.
It appeared that moving existing messages from Cyrus IMAP 2.2.x format to Dovecot Maildir is not as straightforward as described in Dovecot Migration Wiki. Neither cyrus2courier nor cyrus2maildir.py appeared to work correctly on our Fedora Core 4 throwing different kinds of errors.
We will probably have to postpone this migration until later, when we find some time to try some IMAP to IMAP migration tool.