Server migration

[Gandalf Corvotempesta]

Hi
i have to migrate, online, a dovecot 1.2.15 to a new server. Which is the
best way to accomplish this?

I have 2 possibility:
1) migrate from the very old server to a newer server with the same dovecot
version
2) migrate from the very old server to a new server with the latest dovecot
version

can i simply use rsync to sync everything and, when the sync is quick, move
the mailbox from the old server to the new server? My biggest concern is
how to manage the the emails that are coming during the server switch.

Let's assume a 50gb maildir , the first sync would require hours to
complete (tons of very small files) do i can't shutdown the mailbox. The
second sync would require much less time and would also sync the email
received during the first sync (but the mailbox is still receiving new
emails)
now, as third phase, i can move the mailbox to the new server (by changing
the postfix configuration) so that all new emails are received on the new
server and then start the last rsync (by removing the --delete flag or any
new emails would be deleted as not existsnt on the older server)

Any better solution?