You should create a new server with the same mbox structure, si that you can easyly connect to it and then create a brand new server with maildir. 
 

Hi!

We need to move all users from one (pretty old) installation of dovecot to a new one.
The old one uses mdbox for users' mailboxes and maildir for shared/public mailboxes.
The new one must be maildir only.

I believe that I can just copy the shared/public maildir structure to the new installation without problems.
Am I right?  Do I have to expect any trouble?  (Set owner, permissions etc...)

The real problem is that we must not use the running, old dovecot installation.  So we are not able to connect to the old server, pull all folders and mails and create a new maildir structure.  Currently, we can't do anything against it.  What we get are the users' mdbox files.

Is there any way to convert mdbox files and structures to maildir directly from filesystem?
Or do we have to build a copy of the old machine (dovecot only, or -maybe better- a vm) and then use doveadm backup?
Or is it ok to just set up the completely new installation, set mail_location to where the new Maildirs will be, like maildir:~/Maildir and then run something like doveadm backup mdbox:/tmp/$user/mdbox -u $user?  Will this transfer all mails and folders or do we have to keep an eye on some specific things?

I thank you very much and

cheers!
-lutzn