Dear Forum we need some help with upgrading our system.

We have two Zimbra servers with Zimbra 6.0.4 network addition setup in multi-server where server A is the LDAP master and Server B is an LDAP replica.

Both servers hold a list in postfix denying specific users ability to send out email to external domains (only for internal sending).
I know that after the upgrade postfix will no longer hold the lists and the new Zimbra server has the ability to block via Milter server.

Our main goal is to upgrade both servers to the latest zimbra version 8.0.3 but we will need to move to a new version of OS as well since we are running both on Ubuntu 8.0.4.

What is the best practice for doing this ?

Should I install two new fresh servers with Ubuntu 12.4 and Zimbra 6.0.4 , do a full recovery from backup and only then upgrade Zimbra ?

If so how should I upgrade Zimbra ? Incrementally from 6.0.4 to 6.0.5 and so on till i get to 8.0.3 or can I skip some versions getting directly to 8.0.3.

Any suggestions ?