Sadie started to step forward, bit the kid drew back the medallion, "OK, not just yet. From what I've read, you'll get some of my memories, but not necessarily all of them. Upstairs, in my room, is a computer. Password is 'horse'. You might want to make that more secure later. There's a file I called 'term paper on china tea trade'. In that is actually everything I could think to put in about my life. My friends, events, passwords, classes, everything."
"Try and live my life as normal. And hang onto the medallion. Don't let it go too far away. If you lose it, we're both stuck," he shrugged, "I guess I could think of worse things than being stuck as a horse, but I'd prefer the option of being human again."
With that, the kid stuck out his hand. Sadie walked over and covered the other end of the medallion with her lips. They hesitated a minute as nothing happened until they realized that the medallion had to be worn. The kid put the medallion on the horse (so the person wearing it after the transformation would be the human) and prepared to touch it.
Which would have worked if the stable hadn't been full of flies and a fly landed on the medallion before he could touch it. He didn't notice until he touched it and saw that he wasn't changing, but the fly was becoming an odd lump of flesh and the horse was shriveling up.
"Argh!" he said. "I guess a fly is an animal. I could take this off of you but if I did you would be stuck as a horse who's part fly. I don't even know if you could live. I don't think we have a choice." He watched as the horse became a fly and the lump of flesh fell to the ground, developed legs, and turned into a rather confused horse who tried to fly unsuccessfully before it developed a more horse-like attitude a few minutes later.
Then he said "Follow me. You can land in my room somewhere. A fly can't wear a medallion so the only way you can change back is if you swap with something that can. So tomorrow morning I put the medallion on the horse-fly and you land on it and swap again. Then I wait another 12 hours after that, then we go for the original plan."