“Usually, you’ll have the same blood type all of your life. In rare cases, however, blood types can change. The change usually relates to unique circumstances, such as having a bone marrow transplant or getting certain types of leukemia or infections. Not all of these changes in blood type are permanent.”

— https://my.clevelandclinic.org/health/treatments/21213-blood-types

"There have been occasional case reports of ABO blood group antigen change in malignant conditions. We report two such cases of ABO antigen alteration associated with acute myeloid leukemia. These patients had suppression of their blood group antigens during their leukemic phase, and the antigens were reexpressed when the patients attained remission."

— https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5242122/