Warren Shepard was a pretty ordinary teenage boy, or at least, he seemed that way to most people who knew him. He had a few good friends at school but was otherwise quite a quiet kid, got average grades and never got into trouble. Most people never really noticed him.
But deep down, he had a secret: he hated being a boy. He never talked about it to anyone since he knew most people probably wouldn't understand, but often he wished he had been born a girl, feeling like he'd have been happier if he had. Even though he kept this very much to himself, sometimes he got really bad dysphoria because of it, and tonight it was especially bad, keeping him awake late at night.
So when the meteor passed over the Earth, it affected Warren by making his sex change. He felt it instantly, an odd shiver which raced through him as it altered his conception- the sperm from which he formed had an X chromosome instead of a Y- and this rippled out to the person he was now. A kind of anticipation overcame him, and he quietly got up, turned on his nightlight, stood in front of the closet door mirror and took off his pajamas to see himself naked.
And before his eyes, his whole body reshaped itself as his unspoken wish was granted. He seemed to recede inwards in ways that made him visibly less masculine- his muscles turned more lithe, his figure slimmer, his shoulders slanted inwards and his hips flared out, his hands and feet got more petite, even his neck narrowed and his larynx shrank, making his voice higher-pitched. His skin smoothed, much of its hair receding while on his head his hair lengthened to below his shoulders, longer than he had ever worn it. He squeezed his eyes shut for a moment as a wave of sensations ran through his face, and when he could open them again it looked much more girlish, with puffier lips and a softer jawline.
Now came the most integral changes to make him irrevocably female, as the strange sensations honed in on Warren's chest and crotch. He swiftly had an erection, but what stood out to him more was feeling his nipples become erect. His chest felt sensitive in a way he'd never felt before, and when he rubbed one nipple curiously a jolt of pleasure shot through him. His heavy breathing in response to the feeling was accompanied by the flesh around them expanding to form soft breasts. Having breasts didn't arouse him so much as it made him feel content, like these should have been a part of him and he felt better for the fact they were now.
By now the penis and testicles between his legs really stood out as the only masculine part of him- and the only thing making him feel attached to his old self. To his relief, they now started to transform. His shaft receded away while still feeling just as hard as ever as it transformed into the clitoris it would have developed into without a Y chromosome, while his testicles moved up inside him and reformed into ovaries, the incalculable number of sperm cells they contained rapidly binding together to form a single ovum. Something about this made the transformation click in Warren's mind, and she embraced her new gender and name- she was Wanda, and she was a girl. Just to confirm this, what had been the boy's scrotum shrank inwards and a slit formed down the middle, making Wanda's vagina.
Wanda looked over herself in delight- she really was a girl now. She went back to bed, and peacefully fell asleep.
The next morning...