Some blood types are compatable with others. For instance if the kid was AB before the change he is compatibile with all blood types. It just depends on the original blood type and and the new blood type.
This link gives a pretty simple explanation if you're not already familiar
Yeah I researched somemore...He could have had A or B recessive. Just an odd occurance. Mothers who are RH Neg and babies that are RH Pos at birth have blood transfusions to match their mothers. So I will admit I was wrong. Just seemed to set off bells for me is all.