This now supercedes what was answered as a Mailing List question.
"Metamorphosis":
If you activated "Soul Exchange" targeting your opponent's monster, if the monster is face-down, you cannot Tribute it as a cost for "Metamorphosis".
So no more worrying about "fizzling". You as the player must know the level of the monster in order to activate Metamorphosis (so that you can't activate it when you don't have the right fusion level).
"Metamorphosis":
If you activated "Soul Exchange" targeting your opponent's monster, if the monster is face-down, you cannot Tribute it as a cost for "Metamorphosis".
So no more worrying about "fizzling". You as the player must know the level of the monster in order to activate Metamorphosis (so that you can't activate it when you don't have the right fusion level).