So why the change in Cooper when he goes to Judy's Diner?
My theory is that Cooper and his doppelganger were always supposed to merge rather than one annihilate the other but one of them had to die to spiritually enter the other's body. The lovely Philip Jefferies had a genius plan for making this happen. After his attempts to kill Bad Coop had failed he gave him the coordinates to a rock which would turn him into electricity and get Good Cooper to stick his fork in the plug socket around the same time to absorb him. This is a lot less messy than trying to put a ring on a doppelganger surrounded by woodsmen.
People seem to have decided that not all of Cooper came out of the plug socket so