The assignment isn't so much about changing things on your plot points inventory, it's more about updating it. If you feel you've got a firm grasp on what your plot points are and they don't need to change, then just expand on them. Sounds like drawing maps and pictures of important locations in your story fulfill the requirement just fine. And, like Eric the Fred said, as long as you've spentspent the 15 minutes on it and feel that you've fulfilled the assignment, you're looking great!

~Robin