No, seriously...Jon turns into a girl.

"Somewhere on the Internet, someone with a fetish for hamster-sized purple elephant transformations is very disappointed right now," muttered Karyn.
"What are you on about?" demanded Jon. He was having some difficulty talking on account of the fact that he couldn't move his jaw.
"Never mind; I guess it was our narrator's pathetic attempt at being funny. Anyways, where was I? Oh yes...I wish that Jon would turn into a girl."
Jon screamed as the agony of transformation siezed him. His skeleton contracted down to female proportions, his body fat shifted from his stomach to his bum, hips and thighs. His facial features softened and feminized and a pair of D-cup breasts swelled-up rapidly upon his chest; a tangle of long blonde hair poured down from his scalp, and his clohing morphed into a tanktop and a very short skirt, with panties and a bra underneath.
The transformation was complete; Jon had turned into a girl.
"You can move now, Jon. But I wish you wouldn't try to attack me."
Jon crawled to her feet and caught sight of herself in the mirror (which, as it turned-out, happened to be hanging on the door to her closet).
"Why..." she moaned, taken by surprise by her newly high-pitched voice. "Why did it have to hurt so much?"
"Because instantaneous transformations just aren't as exciting," said Karyn appologetically.
"Damn that sadistic narrator!" Jon screamed.
"At least he didn't do that thing where he made your penis shrivel-up and fall off this time...' Karyn muttered.
Jon gaped at her, horrified.
Karyn winked.
Jon started to cry, tears rolling down her face.
"Don't worry," Karyn said, comforting the new girl. "Chances are that we're in for a few madcap adventures in forced femininity, you'll start to like it in spite of yourself (either because I start adjusting your mood with wishes, or because it turns out that you were just secretly 'that way' from the beginning and didn't know it yet), and it will all end either with your masculine persona getting whiped away entirely by a contrived set of coincidences, or the narrator will just lose interest and stop writing."
Jon buried her face in her hands, her delicate feminine form being racked by girlish sobs.
"You can stop reminding the audience that she's a girl now," Karyn muttered. "I think that they get it."
Then, realizing that it was that kind of a story, she lifted up the wishing rock and said: "I wish that Jon would start to enjoy being a girl."
Slowly Jon stopped sobbing and look up at Karyn, her blue eyes glittering with tears in a very girly manner.
"I think that I'm going to be able to get through this, Karyn." said Jon.
"You don't say..." Karyn muttered.
"There's just one thing: none of the women in my family have such huge breasts, and I dodn't have blonde hair before the wish, so why do I look like...?"
"A male masterbatory fantasy? The coarsest, most stereotypical example of 'theme-park femininity' logically possible?" Karyn asked.
"Well...yeah."
"Generally because you are a male masterbatory fantasy; but in this case it's because our narrator wants to show off how 'clever' he is by satirizing and/or deconstructing the entire genre; as it happens, he personally prefers brunettes."
She patted Jon on the shoulder in an affectionate, sisterly manner. "Now then: let's go to the mall, for clothes-shopping is, as we all know, the sum total of what it means to be a woman."