Sarah had always been mean, but Jon was being downright cruel to her. Karyn thought Jon should enjoy a taste of his own medicine. She found someone who would suit her needs rather well.
She aimed the gun at Jon, and then at her other victim, an Asian woman with model-like proportions. She switched just about everything between the two. The only parts she didn't switch were Jon's face and his genitals.
She noticed Jon lacked his bulge. Of course! Sarah currently had Jon's penis! That wasn't quite the lesson she wanted to teach him. She wanted him to know what it would feel like to walk around looking like an otherwise beautiful woman while sporting a big dick. Which meant she had to find a penis for Jon.
She needed to find another victim.