Sorry, I misquoted/misremembered. Inspiring Presence gives Temp HP, and then King's Touch gives Temp HP. I know one is low-level and only 5 Temp HP and the other is high-level and starts out at up 37.5, but whichever is newest wipes out the other.
I like the ability and the kick-ass flavor of getting to be Gandalf, but the one-time-use nature of it makes me feel less like it's an ability and more like activating a magic item. It's like the Ravenloft holy symbols and weapons turned into a character ability, a plot device and then it's over. Take someone that is an optimizer/min-maxxer and make him a 14th level Grey Wizard, and the first thing he does is jump off a cliff or something to trigger the Metamorphosis.
It would be less true to the Gandalf theme, but if it happened any time you were reduced to 0 HP but the transformation lasted for a set duration (10 minutes or something) then it would feel more like an ability and less...jarring?