What's the REAL method for changing the player's character?

I'm currently making a creeper chaos game, and I want to change the player's character into a creeper. But the problem is whenever I try to change the character using (player.Character = Creeper), it's not working. Then I tried methods I heard from other people, but those ones don't work either! I did this before, but somehow it doesn't work for this particular occasion. So any methods for this that anyone else has will help a lot.