So, just did this at an FNM, and I want to make certain I did it right. I had {Mutavault} on the field. I activated it to be a creature. I then double {Rancor}'d said creature. At the end of turn, I assumed since the Mutavault is no longer a creature at the end of turn, the Rancor's go back to my hand. Is this correct?
Correct! They will fall off once it's no longer a creature, and go to your hand.
Awesomes. :D