Why does the blood not clump and result in death? In this question's accepted answer, it is said that the blood type will slowly change to that of the donor's. When the blood in the person is about 50% his own and 50% that of the donor's. i.e, 50% A group blood (his own) and 50% O group (donors blood),won't the antibodies in the O group react with the A-antigens and clump?

Some people do die from graft vs host disease after bone marrow transplants involving ABO incompatibility, but not usually from blood clots.

Foreign red blood cells don't usually form significant clumps (or clots) as a result of an antibody interacting with a surface cell antigen. They can form tiny clumps, however, that are not life threatening, but simply plucked out of the bloodstream by macrophages.

The typical reaction consists of foreign red blood cell detection and destruction (broadly called hemolytic anemia.) So anemia is a problem, but normally not significant clumping/clotting.

Safety and impact of donor-type red blood cell transfusion before allogeneic peripheral blood progenitor cell transplantation with major ABO mismatch.
Blood transfusions and the immune system
Consequences of ABO incompatibility in allogeneic hematopoietic stem cell transplantation.