Not sure if I'm completely right here, but here's what I think. When you retake a class you better get an A, you've already seen that material and have an idea of what is being tested on! Post bac is at the graduate level, so the classes should more difficult, time intensive, etc., than undergraduate classes.
Side note: Remember only DO schools will replace a retake grade, MD schools will essentially average the two grades. If you received a C and then later got an A is the same as getting two Bs for MD schools. So in that sense, it makes more sense to just move on and do well in other classes (undergraduate or graduate) to boost GPA.