We're not LPARd (yet), but we do have 2 systems.
We upgrade the 'test' system first, let it age a bit to find bugs, then upgrade the 'live' system.
The test system gets a lot of workout from the developers, who are pretty good at tripping over the bugs!
Our auditors "encourage" this type of behavior, and it does help prevent "oopses."
Paul E Musselman