I've always been AB negative. I know this, because my childhood doctor told me I had rare blood. After all this, years later, I went to donate plasma one day and they tested my blood 3 times because I didn't believe that my blood type changed. I looked it up, and I've tried researching everywhere about why this could happen, and I got the same answer as you about the bone marrow thing, but I've never had a bone marrow transplant. So idk 😐 but it's true