You have a bunch of nerve tissue around your body, which contain information. Your brain is also calibrated to your organs. Your brain may notice you need to

You have a bunch of nerve tissue around your body, which contain information. Your brain is also calibrated to your organs. Your brain may notice you need to release X amount of adrenaline, and sends out a signal. Then your new body releases a different amount, because your new adrenal glands are used to a slightly different signal intensity. And the body contains an enormous amount of complicated metabolical processes which are tightly coupled together. Imagine the chaos, if everything starts to work a little differently, because “there is a new boss”. I have no formal education in this yopic and my comment is only speculation, but it seems logical.