The big problem with a head transplant is making the body work after the head is attached. They've been sewing animal heads on new bodies all holus-bolus since the 1970s, but the resulting frankenpoodles were all paralyzed because reattaching the spinal cord is hard as shit to do. Or it used to be. Because thanks to research by the Turin Advanced Neuromodulation Group, the problems with reattaching that spinal column have been overcome, and now we can all swap noggins!
"I'm a donor. My finger, my butt, your nightmares."
OK, maybe it's not as simple as all of that, and the procedure hasn't been tested yet, but the science is there. The terrible, creepy science.
Say you're in an accident in some kind of industrial head separatoring machine like they use in the back of Walmart or whatever and oh shit, there goes you head. As is tradition, your body is turned to mulch and you're in a bit of a bind. Luckily, a guy across town was also in a head separatoring machine, but he went in upside down and his head is now a pudding. So you and him meet up at the hospital and a doctor puts you two crazy cats together. Now when you wake up, you have creepy new hands that you're afraid to pick your teeth with, and when you poop, it's probably not a color you're used to because those guts aren't yours. And your crotch is a whole new crotch that probably isn't pleasant to you because how can you like having new genitals? What if your new junk is bulky and weird? What if it smells funny? What if there's a mole on it? How will you ever handle having a new body?
If the new body heebie-jeebies don't get you, then taking a moment to think about tissue rejection ought to scare the shit out of your new ass. What if that body rejects your head? That's literally the most awful thing I can think of. Do you want your head going necrotic? It's your head!