You can't change them right this second, but if you want to change once you get to campus, you can do that -- once class starts, you can add the class you want using an add form that you can pick up from the registrar's office. You just take it to the first day of class of the course you want to add, the professor signs it, and you turn it in.
I didn't take 24.900 with Richards, but I've heard he's good. If you want to see his previous ratings when teaching the course, they're
here (certificates required) -- they're almost certainly more reliable than ratemyprofessor. When I took 24.900, I contacted him for help with my final project, and he was very helpful and nice to me.