Migrate a licensed Server

Hello, we need to upgrade our Zimbra open source server (with licensed zextras) and change the underneath OS. So we would migrate the actual server to new platform.
the point is that we cannot use the incremental migration (our DB is huge) because the trial version of the zextras does not import backup older than 30 days.
What can we do, without making a new backup of course? can we use the same license to migrate the data between the two servers? how?

davide