Do you have a link to the larynx transplant paper? That sounds fascinating. I was actually more thinking about very understood practical lines like I know my personality is drastically different (according to friends) on certain medications. With the role even common hormones like testosterone have and the various feedback loops that could come down to the genetics of body cell lines, I can't imagine a person's personality not changing, at least in a time/environment dependent way. Seasonal depressive disorders come to mind, as those seem to have partially somatic causes.
Another interesting question is to what degree does the body (i.e. muscle memory) assist or modify how memories are "pulled up" and processed? Perhaps there will be random unavoidable "corruptions" in the fusing process that result in the signals for stimuli changing (i.e. sensitivity to heat and cold is probably somatic) and the brain pulling up inappropriate emotions or responses?
Is it still your identity if chemical mismatches change your interpretation of your memories over the long term or are you now a hybrid that, due to the transplant, has the extreme fortune of experiencing life and even epistemology in an entirely different way?
Perhaps the new body does feed back some changes into the brain, but overall we still have a cortex with the original memories, etc as a starting point.