"The skull pendant grants wishes," Karyn explained.
"Oh. Then I wish you would go find your own boyfriend," Tiffany said, with a smile. She always hated Karyn for hanging out with Jon. Jon should be with his girlfriend, not Karyn.
Which normally would have worked. The skull pendant reverses wishes made between the wisher and Jon. Karyn was a third party, so a wish about Karyn should have been granted as made.
Only... Karyn had some feelings about Jon. If there was anyone she wanted to be her boyfriend, it would be Jon. Which meant that the envy part of the pendant did work after all. So instead of Karyn's boyfriend being Jon....
Tiffany felt something twisting her arms and legs. She looked at herself and saw there was nobody there--she was just changing! And it felt like she had a stick between her legs. "What are you doing to me!" yelled Tiffany in her new male voice.
"I tried to warn you," said Karyn. "If you wish something for yourself, Jon gets it and vice versa. Instead of Jon being my boyfriend, you are!
"Change me back!"
"I wish Tiffany knew what was going on here!" A wish which worked as given since it wasn't about either herself or Jon.
"You have got to be kidding me!" said Tiffany as the knowledge filled his head. "You mean I wasn't Jon's girlfriend?"
"You weren't?" wondered Karyn. "I remember it like that but I guess it was a wish. But you're my boyfriend now..."
Tiffany thought and suddenly he knew what his new name was. "I'm Stefan now... but I'm not really your boyfriend! It's just a wish. I just want Jon, is that too much to ask?"
"I don't know. Why do you worry about Jon when you have me?"
It was hard for Stefan to deny it. He too felt as if he was Karyn's boyfriend, even though he still remembered what had happened before....