Two possibilities

"You turn me into a sexual freak... happy ?" Yell finaly Sarah.
"Not quite." Answer Jon "I have to save Karyn... but I have an idea..."
"Oh yeah ?" Says Sarah with an agressive tone.
"I wish Sarah and Karyn exchanges bodies."
Since Sarah only made physical changes to Karyn (even the stupidity is neuronal) and Jon made only mental change to Sarah, its a winning combinaison.
The next moment Sarah look at her body.
"What ? ... What happened ? I don't feel dumb anymore... and why am I in Sarah's body ?"
Says Karyn in panic.
"I exchanged your minds so now Sarah is a dumb bimbo slut with bondage and exhibitionism fetish, and you you're almost you... Now I have just to wish..."