You pull out a rather opulent looking on-the-shoulder dress. You look at the label and find that it's from the Philippines, and after a little research you find out it's called a terno. It's kinda like a ball gown, and you have to admit even if you're a guy it does look spectacular. After some consideration, you decide to try it on.
After undressing to your underwear you undo the back of the terno and try to step into it. As you start to pull it up, you feel it tightly cling to you and worry that you're gonna rip it. But thankfully it seems to fit your body more neatly as you pull it up, and you seal it, walking to the mirror and smiling as you see the brilliant white of it and the vibrant multicoloured flower pattern that stretches from the neck to the thigh. It's a bit of an awkward fit in the chest for obvious reasons, but otherwise you're really happy with it.
You look better in this dress than you ever expected any girls' clothing would make you look, especially one probably made for a woman smaller and thinner than you. Satisfied, you reach behind you to undo it, and a weird brief shiver races through you as you find the zipper, undo it and start to pull the garment off. Once you can see your bare skin again, you notice it's become almost totally smooth and turned a sort of darker, tanned colour. Your body and muscles also seem more slim, and your nipples are more of a brownish tone than a pinkish one.
They feel strange, and you let out a choked, high-pitched sound as they pulse and start to swell and widen before your eyes. The flesh around them enlarges too, and you realize you're forming breasts. The new weights on your chest feel weird, but at the same time weirdly right. They're... pretty.
Once your boobs finish sprouting, the weird sensations move downwards, and you drop your underwear to see your penis- you'd almost forgotten you had one. And as it is slowly tugged back inside your body, the sac between your legs getting deprived of balls and receding to a mound, it doesn't feel as wrong or weird as you'd expect. Soon your phallus has become nonexistent and formed a clitoris, your testicles have moved up inside you and formed ovaries, and your scrotum has re-separated into labial lips and a vaginal slit has opened.
The boy you were before barely feels like he existed, and you feel embarrassed to see your naked female body so shamelessly out in front of the mirror. You hastily pull up your terno and scurry off back to your house, knowing at least you have the slumber party to look forward to.