You can follow the steps I had to go through on: My Blog.

the steps to get my original machine running CentOS 5(64) to Centos 6.4 was just too long. The physical upgrade took about an hour and a half.

I upgraded from Zimbra 7.1.3 to Zimbra 8.

Thank you to everybody who wrote in the forum regarding upgrading to Zimbra 8. Without them I would never have made it.

A BIG PS: I did not migrate from a backup, I migrated directly from the live system due to the fact that I ran out of Backup Space on the original server.