"I am a girl," Noah had written. He had always found the idea kinda interesting, he wished he knew what it would've been like if he had been one, but it could never happen, right?
Wrong. The world around him seemed to vanish into blackness, and Noah shut his eyes for a moment. When he opened them again, he realized he had come to an endless void. A strange vision appeared before his eyes- an egg surrounded by sperm, one of which moved inside it and fertilized it. But then time rewound, and instead a different sperm pushed into the egg first and fertilized it.
Noah realized as a strange shiver ran through his body, that was his conception. Or rather, her conception. The journal was gonna turn him into a girl. The surface of the void turned reflective, and he watched and felt his body be reshaped by the sperm that had reconceived him.
His old self gave way to her new self without any hesitance. His face reshaped to have softer cheeks and puffier lips, and his hair lengthened partway down his back. His larynx shrank and he developed a higher-pitched voice. His shoulders slunk inwards and his hips expanded outwards as his arms and legs turned from faintly muscular to lithe, and his hands and feet became more petite. His skin turned smooth and almost hairless all over.
Once the changes reached his chest and crotch, it truly got through to him that she would be female now. His nipples swelled up and grew more sensitive, and around them mammaries formed and soft flesh pushed out- he cupped them with intrigue and his breath hitched at the sensation. Between his legs the shaft of his penis receded away until all but the head had been subsumed into his body, while his testicles moved inside him and reshaped into her ovaries and his scrotum flattened itself out. A womb formed inside her, and his cock head turned into her clitoris while an opening formed in his scrotum and it became her vagina.
While his body finished turning into her body, in his head the changes helped him adopt to her new self. Now Noah was Noelle, not he but she, not male but female, not a man but a woman. She remembered her life as Noah, but the similar memories that flooded her head of being a girl all this time were the 'real' ones now. Not only that, but some of her instincts had changed too- she was still heterosexual, but instead of being attracted to girls, she felt attracted to boys. Noelle blushed as she thought of a handsome man, as if to test that, and her nipples grew hard and she moistened between her legs.
Noelle shut her eyes and then found herself back in her bedroom. To her relief, she was clothed, though in a strappy top and skirt instead of the T-shirt and jeans he had been wearing before he changed. She looked at the journal in amazement, and wondered what to do next...