The Atomic Brain (1964)

Mrs. March, sadistic, selfish, and rich, uses her aging gigolo boyfriend and renegade scientist Otto Frank, whose illegal experiments she is subsidizing in her Gothic mansion, in a plan to transplant her brain into the body of a young woman. Three foreign domestics: Mexican Anita, British Bea, and Austrian Nina, are hired. Frank's mysterious and unexplained use of radiation can inexplicably transplant brains without surgery. After he puts a cat's brain into Anita and she literally scratches out Bea's eye, Nina becomes the prospective receptacle for Mrs. March's aging but still active libido.

AKA.
Monstrosity