[user104658]

No it is the full head + brain... Basically putting some ones head from a "broken body" onto a working body. Like a limb transplant but with the full thing.

Spinal cord reattachment is possible in ideal circumstances, I guess it doesn't work very often after accidents because the cord has been randomly "torn" rather than surgically severed on purpose in just the right place? That would be my guess, anyway.

It's definitely an interesting one because if donor rejection which is linked to the immune system... But the immune system I think is within the body rather than the head so it would be the head that was rejected rather than the donor body. Having your immune system attacking your head and brain would probably be pretty unpleasant .