I'd go with the above method. After the upgrade, I'd add an additional partition that I'd use for the new /opt directory. I'd shut down Zimbra, move the data over to the new /opt and remove the old one.
1. Upgrade Zimbra 6 in our current virtual machine, expand existing partitions and add additional partitions. Move varies Zimbra folders into other partitions.
I'd probably use rsync to do the data move.
Ben Franklin quote:
"Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety."