[Dovecot] Maildir migration and uids

I'm in the process of migrating a large number of maildirs to a 3rd party dovecot server (from a dovecot server). Tests have shown that using imap to sync the accounts doesn't preserve the uidl for pop3 access.

My current attempt is to convert the maildir to mbox and add an X-UIDL header in the process. Run a second dovecot that serves the converted mbox. But dovecot's docs say, "None of these headers are sent to IMAP/POP3 clients when they read the mail".

Is there any way to sync these maildirs to the new server and maintain the uids?

The real goal is keep customers email clients happy when they are pointed at the new server. Am I just wishing?