You're not sure you're quite ready for this kind of experience, and decide to look around the mall some more before blowing most of your money on a costume. You take one last look at your bare female form, and then begin donning your old clothes.
The boxers and shorts are terribly loose on you, and the shirt is much too baggy. You have to keep at least one hand on your waist to hold the shorts up, and your neck looks comically thin sticking out of such a wide collar. You feel as if taking just a few steps in your giant shoes will cause you to walk right out of them.
This clearly isn't going to work. Your younger, girlier frame can't accommodate your old clothes. It looks like you're going to have to put on the costume after all.
You're about to disrobe again, when you suddenly feel your clothes shifting on their own. The shorts suddenly become tighter, and you can feel the boxers within shrinking to hug your crotch snugly. You see your shirt shrink around you. Your breasts are suddenly hefted up, and you gasp at the feeling of something binding them closer to your body. Feeling the fabric of your shirt, you can tell something is strapped around your torso. You're certain it's a new bra. Your boxers have probably turned into panties as well.
You begin to see more drastic changes occur. Your plain white socks are not only skin tight now, but they're growing up your legs. They finally stop at your thighs. Your sneakers shrink and smooth out to become polished black pointed shoes with an inch-high heel for a tiny lift. Your shorts lose definition, the pockets and middle zipper area disappearing into the smooth fabric as it turns a black color. Finally, your shirt transforms into a white button-down blouse with long cuffed sleeves and a fancy collar with a bit of lace trim.
You look at your reflection in shock. Somehow your own clothes have become a costume similar to the school uniform...though these are clearly civilian and not tied to an institution. Does this mean you can leave the store on your own after putting away the real costume? Do you really want to stay a girl, or do you want to look for a way to change back before anything stranger happens to you?