mailbox / user migration to a new server

Folks;

we're in the process of moving an "old" (cyrus 2.2.13, local Unix users) 
IMAP structure to a "newer" installation (cyrus 2.4.18, ActiveDirectory 
users attached using winbind). In total I'm about to move somewhere next 
to 300GB of mail spool knowing that ...

- ... all this data has to be moved from one server to another and

- ... most of the users now do have different user names (as the "old" 
Unix names differ from the "new" AD names) so data needs to be copied 
between different mailboxes.

So just copying /var/spool/cyrus won't possibly work. Likewise, however, 
I can't use any of the IMAP migration scripts that do these things on a 
per-user basis as I am not supposed to know or mess with each users 
passwords; whatever I do sync-wise should be done using cyrus / 
administrative user accounts.

Is something like this possible? How if so? What is the best way of 
doing this?

TIA and all the best,
Kristian
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