The costume in question looked like little more than a loincloth, a halter top, a pair of boots, and a bow. And it was all obviously sized for a rather slender woman. Still, looking at it, you couldn't help yourself. Stripping down, you pulled on the different pieces and stood in the middle of the tent, feeling utterly ridiculous.
Then all at once the changes began. Your hair spilled down your shoulders, fading from brown to a start silver as it grew down to the middle of your bare back. You shuddered with pleasure at its touch as you limbs and shoulders narrowed, while your skin became creamy, pale and smooth. Beneath your loincloth you felt your hips broadening into pleasing curves as your bare legs became shapely and muscular. You ran newly long and slender fingers along them, relishing the feeling of taught muscles under soft skin. Your face was also changing, its features becoming more angular and lovely, with a slightly inhuman cast. Then your ears tingled sharply before elongating into a pair of sharp points; suddenly the sounds from outside the tent became much sharper. Your other senses were also enhanced; you could see everything in the tent in stunning detail, while your nose was picking up scents you'd never before imagined.
Beneath your halter your nipples stood to attention as the flesh beneath them swelled into a pair of small but firm and shapely breasts; beneath your loincloth, your manhood faded away leaving a warm, moist vagina in its place. Tattoos of vines and leaves curled up along your arms and onto your face, and then it was over. Panting, you stood up and faced the mirror in the corner of the room, where you saw a lithe, lovely wood-elf maiden staring back at you. You couldn't help yourself; you smiled and preened.
You remembered your human life, but you had other memories as well, now - you were Seriande, elven ranger, and knowledge of language, skills, and a whole other life you'd never lived filled your head. Grinning, you bent down and picked up your bow and quiver and walked outside with an easy grace, swaying your hips seductively, to where the girl who'd recruited you for this adventure was waiting.
"My, my," she said. "You turned out better than I could have imagined."
"I know," you said. "Let's go have some fun."