One careless wish deserves another

That evening after dinner, Jon had the stone out on his desk. He was staring at it, trying to think how to help his friend Karyn. When inspiration struck, he called her up and told her that he might have figured out a solution. Karyn arrived half an hour later. Once they were up in Jon's room, he held up the stone and said, "I wish that Karyn could change her shape." Karyn frowned and asked, "Will that actually work? I mean, would that change the wish I already...oh!"