If I retake a class, is my old grade replaced?
No, this is a big misconception among pre-med students. For all medical schools under the Association of American Medical Colleges (which is almost all MD medical schools), ALL classes taken for a letter grade will be counted in the GPA that medical schools will see. For example, if you received a F in organic chemistry and then retook it and got an A, medical schools will count the F and the A in your final GPA.
Some students believe it is a good strategy to purposely fail a class if they no longer have a chance at getting a good grade. Please do not follow this advice.
DO medical schools have a different policy. They will only count the last instance of a repeated course.