Attending lectures for a course I already passed? Two years ago I took an upper undergraduate/graduate course which I passed with a good grade. Sadly I missed many of the lectures, did few of the suggested exercises and managed mostly by cramming a few days before the exam. 

To little surprise but to my great regret, I have now forgotten quite a lot of the material. I cannot formally retake the course, but I have considered attending the lectures and work through the exercises, while not handing in any work. Would the professor, who taught the course the last time I took it, frown upon this? Would he question the grade he gave me and or consider me to be a poor student for forgetting so quickly.
At least I would not discourage anyone from sitting through a course again. The notion that one has permanent and total recall of almost anything is untenable: one must review and rethink (or use) periodically, or things fade away from disuse. For that matter, there are usually several layers of understanding/appreciation, and getting to the subtler ones takes time and (re-)thought.

Perhaps you'll be better off than others who got their grades and don't even realize that they'd benefit from re-thinking the material.