I need a migration with no downtime

I want to migrate my actual zimbra 7.1(rhel 5.5) to a new fresh server (Linux rhel 6.4) zimbra 8.0.6.

Where should begin? I have a large amount of data to migrate (300 gb) and I don't want to lose or put in risk the normal operation of my production server.

Any help would be appreciated.

Thanks