Blood group is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrates, glycoproteins, or glycolipids, depending on the blood group system. They are determined by a set of genes and thus, once it is surely determined, the group does not change. There is one possible way for a person to change blood type—but not spontaneously—and it requires a major medical procedure known as bone marrow transplantation.
It is more likely that the grouping done either earlier or at present is not correct. So please get the confirmation of the grouping done from a good lab.