Biohazard (1977)

The former head of the Institute of Science - Professor Molnar, returns to the Institute to seek treatment for his heart condition. The new director of the Institute, and his former assistant, Dr. Egberg had a falling out with Molnar after the latter considered his research unethical. However, after waking up from surgery, Molnar makes a horrifying discovery; Egberg has swapped their brains!