I retook it and didn’t regret it (my old course was 12 years old). I thankfully didn’t have to take the MCAT (thanks to my postbac’s linkage program), so can’t comment on how helpful it would be for that; but I can say that my postbac had a particularly strong (and tough) premed bio course that really made a difference in my first year of medical school – in addition to having freshly studied many of the topics, the bio course strengthened my ability to quickly learn a massive amount of details and how they fit into a larger system. The way I studied for bio is much more similar to how I study in med school than how I studied for orgo/physics/gen chem/calculus. But then, I didn’t take any upper level courses, which may do the same thing.
Is there an add/drop or “shopping” period at your school? Maybe sign up for the upper levels, go to the first class or two, and if it’s over your head, drop it and take the intro course.