How exactly can it happen that your blood type changes?

I have recently shared a story about an O positive patient’s blood type changing to O negative and an O negative patient’s blood type changing to O positive.
When examining possible reasons, a simple method of converting blood from one group to another comes to mind:
The process uses bacterial enzymes to cut sugar molecules from the surface of red blood cells.
A study reported on a 4-month-old girl with congenital rubella who had type A blood that eventually switched to type O after weeks of testing. The scientists suspect an enzyme just “ate” the type A antigens, which made this little girl’s blood type appear to be type O:

An infant with probable congenital rubella infection developed altered blood group expression. This was noted at 4 months of age. The child’s blood was typed on seven separate occasions during the first 8 weeks of life as type A, but on repeat testing, her cells failed to agglutinate with anti-A and anti-A,B typing serum. The A antigen was present, however, because an eluate made after incubating her red cells with anti-A contained anti-A, and A antigen was demonstrated on buccal mucosa cells. Altered expression of blood group A (loss of agglutinability) has occurred previously only in association with hematologic malignancy.

This could explain also an rh positive person becoming rh negative, but not vice versa.
More to come …