If you think of a person as a bunch of memories loaded into a certain brain architecture, then of course the person would change in a new brain architecture. The architecture is as much a part of us as anything else we thing goes into making a soul.
In my opinion, most people use the term as a place holder for what is a person distilled down to their essence. But a person is a person only with their brain architecture and their body. Without the hardware, the person doesn’t exist. We think in our bodies. Not just our brains. We can not actually separate brain from body. Our whole selves depend on the physical apparatus.
So what happens to the soul after death? There is no physical soul, so all the physical stuff degenerates quickly after a person dies.
Then what is the soul. Soul is the essence of a person. Normally it is held in two places: the person’s concept of themselves, and other people’s concepts of them. When the body is gone, the soul only exists in other people’s minds. It lingers there, slowly getting less and less as the people who knew you die. Eventually your soul has disappeared entirely because there is no one left to remember you.
For a few, the soul lingers on a bit longer in the information that has been recorded about them. This happens in all media—books in the past and now in recordings of images and video and audio. Those memories are recorded there and any time someone takes in some of the material, the soul of that person increases in vitality. For some people, like Moses, or Mohammed or Buddha, so many people remember them and the memory is continuously being renewed, that their souls are great and nearly permanent.
Brain chemistry is essential for the creation of a person and that person’s soul. Without brain chemistry the person is dead and there is no soul. In fact, though, brain chemistry isn’t special. Every part of a person’s body is essential for that person’s existence, and they must exist in order to have a soul. If they lose a part of their body, or if their brain chemistry changes, the soul changes in lock step. Brain chemistry isn’t special in this way.
I suspect you are asking a different question, though. I’m not sure, but it seems like you may be interested in knowing the effect of a change in brain chemistry on personality and sense of self.