"...plain soda?" You decide not to overcomplicate things.
The young woman pulls one from the refrigerator, passes it to you and smiles, "Here you go, Elizabeth! That'll be 4 dollars." You hand her the money and smile shyly. "By the way, I should just explain how it works. One sip changes you for 12 hours. Half the can, 24 hours. If you drink the whole thing, it's permanent. Well, until you drink the same flavour again!"
"Sounds good," you say, smiling shyly and taking a swig. Then another, and more and more... you can't tell if it's because of the effect you know it'll have or the actual taste, but either way you love it.
Once you finish chugging it, the young woman laughs, "The bathroom's right through there! Enjoy the show." Right on cue, you feel kinda weird and rush in there, shutting yourself in a cubicle.
The door has a mirror on it, and in it you get to watch yourself relinquish manhood for womanhood. You hastily pull your clothes off, and they all seem itchy and uncomfortable against your skin as it softens and turns more supple. Once you're naked, you notice your body hair has mostly vanished and you seem smoother to the touch.
You burp, and then your face convulses; between your eyes screwing shut and reopening, your features have subtly reshaped to be way more feminine. You gasp and hear your voice's pitch increase as your larynx recedes, and even your shoulders slink inward a little.
Next things move downwards, and your arms and legs turn more petite and lithe while your figure goes from straight to curvy. You get an erection at how feminine your body is, and wince at the dysphoric sensation.
Thankfully, the changes now reach your reproductive system, reversing the course it took while you were in the womb. Your nipples turn wide and puffy, and mammaries form under them, soft breast flesh developing until it reaches a C-cup.
As your penis and testicles start to change, you feel yourself ejaculate, the last of your sperm squirting onto the floor. Your balls then clench upwards, and get tugged inside your body with two wet pops, instantly starting to reshape. Your castration is followed by emasculation as your phallus and scrotum recede into your crotch, the shaft turning into a clitoris and the sac separating into vaginal lips. At the same time, your newformed ovaries link up to organs forming inside you, as your womb and vaginal tract develop and link to your crotch.
It's done. You slowly recover your senses, and gaze in amazement at the young woman you've turned into now. You're female, inarguably so, and you feel so pretty and so happy about yourself you could cry.