My question is, how do you get the creeper to convert by itself?
It was a mistake at first, actually. I never meant it to happen.
It's a glitch in the game.
When you create creeper in the editor and max it out, it will go to a maximum of about 2100M. You just cant create a higher density.
When you force the density up (by fields or gravity), it goes over the 'critical point' and changes to AC.