> Of course we don't even know whether this sense of self is really purely attached to the brain. That brain is still connected to the rest of your body, and who knows what the body chemistry of the body donor will do to the new brain?

Imagine just one aspect of this. A thyroid disorder.

If the donor body was hyperthyroid, the head transplant would result in someone with the symptoms of hyperthyroidism. Angry, irritable, racing pulse, etc. If they had hypothyroidism, they would be depressed, lethargic, etc.

And that's just one organ in the body. It could radically alter the "person" who they are presuming lives only in the head.