In-vivo, the blood group of a person is dictated by genetics and cannot be changed, according to Dr. Mehmet Oz, a professor of surgery at the Columbia University. The reported blood group changes are likely caused by a testing error during the initial test.
Scientists found evidence for blood’s capability of changing group type when acted upon by certain bacterial enzymes, the Telegraph reported. This lead to the development of a groundbreaking in-vitro method for converting group A, B, and AB blood to group 0 blood.
Dr. Oz mentions the possibility of a similar mechanism acting within the human body, leading to altered blood group expression. However, this isn't an actual blood group change.