All I need to do to change mine is to fill out a form, I've not yet had the necessity to challenge a class.
A lot of universities do have rather arcane procedures for challenging classes; I suspect this is at least partly deliberate as a form of deterrence. Given the number of students who think they know material and don't, I think this deterrence is justified.
After all, it's much easier to game a test than a course, especially if you're well-connected. One of the standard ways to challenge a course, for example, is to sit last year's final exam and have the professor grade it. It's standard because it's simple (the materials are ready to hand), it maintains control with the faculty expert where it belongs, and it requires little work on the part of the faculty.
It's also heinously subject to cheating. Simply get a friend or fraternity brother who took the course last year to keep a copy of his final, and have him go in to "discuss" the test results with the instructor some time over the summer. Behold, an instant answer key to memorize!
So, of course, one can establish counter-measures. Write a new test for each challenge, for example. But at this point, it starts to no longer be simple and easy to evaluate a student's claim to knowledge.
... but this just points out the problem with autodidacticism. If it's difficult for me, as a faculty instructor, to evaluate the knowledge level of a single person
in my area of expertise, how much harder will it be for a non-expert to evaluate and compare the knowledge level of a hundred candidates for ranking?
And if someone else -- a third party -- volunteers to do the comparison,... what does that add? Now we've added a third party's worth of complexity and cost, while at the same time adding another point of potential dishonesty, incompetence, or simple failure. We've gone from a simple system that works pretty well to a more complicated system that works more poorly
... and all because some whiny little undergraduate brat can't be bothered to get out of bed for his 9am section?