Zaps his new body with it

The Katie mind in the David body is flooded with a lifetime of David's memories, and the Katie self finds it has to concentrate in order to think or act like Katie's: behaving like David has become the default.

The expression on the face in the mirror is a purely David expression, and the body language of the transformed body is David's.

"Yes!"

says the new David, pumping a fist and seeming to himself entirely David and not Katie in the least.

The new David examined his David memories and presently it struck him: the original David hadn't loved Jessica as much as he, the former Katie, did! He was certain now that he'd done the right thing: he was the David that Katie's sister Jessica deserved, not the one who thought of Jessica as little more than a living and breathing blonde sex doll!

Katie and Jessica's parents, and probably Jess herself, would be upset over Katie's permanent disappearance, but that couldn't be helped and maybe he could skin himself briefly now and then to write and send messages that would perhaps convince them that Katie was alive and well.

Only the smug expression on his pleasant face marred the new David's looks.