[AlwaysWrong]

I'd say you are making a huge assumption in identifying the mind with the brain. If you're a materialist, the mind is a function of the entire body, not just the brain. Although the majority of cognition is done in the brain, my consciousness must also be determined by the rest of my body: my hormones, my gurgling stomach, my blood-sugar level, etc.

So my inital response would be that the answer to your question isn't the answer to the question you want to ask.

Moving on...

If you're a dualist then the answer to your question depends on how you think the mind interacts with the body and the brain. It could be that cognition is entirely determined by the mind/soul, and the brain is simply a lump of flesh that relays instructions from the soul to the body and relays sensations from the body to the soul. In that case the question becomes how is the soul anchored to the body? Probably the only way to answer that question would be to perform the transplant and see if switching brains switches behaviour.

Obviously whatever body that is controlled by the soul would be the one whose actions the soul is responsible for when determining if the soul should go to heaven.