Actually, for Midrange systems, Itanium is a very small step up for Pa-risc boxes and a very robust improvement.
For newer Cell-based systems, it is only a matter of replacing the CPU and upgrading the firmware and a few other modifications. If you have an older cell-based box, its better to buy the new hardware... upgrading would be difficult without gutting an older superdome or keystone.
I just had a class from HP regarding HPUX 11i v2 which was very enlightening... 11i v2 (11.23) is a vast improvement over 11i v1.
Of course, Midrange boxes arent good for personal use!