Move dovecot between servers

Hi to all I have a very old dovecot server (1.2.15) with about 195GB used and 1081 email accounts. I have to move this server to another one with contextual dovecot upgrade.

Anyone did this before? Any advice? Obviously, existing emails *must not* be downloaded twice by existing clients (outlook, thunderbird, ...)

I have no access to customer's clients, only to my server.