<p>I don’t think there are many restrictions on math classes (might be wrong though).
You can sign up for whatever math class you want - undergraduate, graduate, honors, etc - except for the ones that you’d expect to have some limitations (honors thesis, independent study, professional, etc).</p>
<p>Though, you might have to take a lower-level course if the major you are going for requires you take one. For example, you probably can’t use Math 202a in the place of Math 1a and you would probably have to take Math 1a (calculus) even if you took 202a (graduate analysis & topology). However, you can indeed enroll in and finish math 202a without ever taking math 1a.</p>