"I guess I'll just go with plain soda?" Edwin replied, guessing there'd be weird consequences if he chose a weird flavour.
"Sounds good!" the girl replied, going to get one from the chiller and then handing it to him, before leading him to a seat. "Enjoy!" she smirked as she went off to the counter, watching him from there.
Edwin's hands were shaking as he pulled the tab from the can and moved it to his lips. If you don't drink the entire thing, it's temporary, he reminded himself. He carefully sipped it and set it down on his table.
The small sip was all it took for the soda to start affecting his body. A shiver ran through him and he breathed heavily as he started to feel... different. He got up and rushed to the counter, saying hastily to the girl, "I've gotta go to the bathroom!"
She giggled and pointed him in the right direction.
When he got there, he noticed they were unisex, and just ducked into a stall. He felt a twinge as his T-shirt rubbed against his nipples, and pulled it off. The nipples were far larger and perkier than before, and looking at them made him feel very weird inside. He started to imagine having someone play with or suck on them. As if in response, the flesh around them swelled, and he moaned in an oddly high-pitched voice as they grew. He looked down to see he had breasts now, groaning as his figure turned curvier and his arms, legs and body became thinner and smaller.
By now, he looked and felt like a girl in all but two respects: his mind, and... he dropped his jeans and underwear to see his hard penis. A second later, he released a surprised moan as he orgasmed hard, his cum squirting from him as something inside started to reshape his reproductive organs. His testicles were sucked inside her, his scrotum was tugged upwards and vaginal lips peeled open at its centre, and his penis shrank rapidly to a tiny nub with all its sensitiveness accumulating in a much more confined space.
She sat there for a moment looking amazedly at her body. She'd really just transformed from a boy to a girl like it was nothing! Pulling out her phone to get a mirror, she took a look and observed that she looked quite like he had- they were clearly the same person, just that one was a boy and one was a girl. In her head, she decided 'Edwina' sounded like a fitting name for her in this body.
Edwina prepared to leave the stall, wondering what to do next...