You are still an Undead Creature when you were first affected by the Spell and Fell Energy doesn't state the effects end if your type changes at any point afterwards.
I mean, you'd literally have to state you were unaffected by Devil's Ego, and thus no Type change, in order to say you were not an Undead Creature when the Fell Drain enhanced Spell effected you. There is no other possible way to state it.