This might be an urban legend, but I always heard when I was in college that the published catalog that was in effect when you enrolled was a contract between you and the university as far as degree requirements; nothing could be added or changed once you had declared your major.
Not an urban legend- that's the definition of 'catalog'. So long as you stay with the catalog, you cannot have additional classes required. Of course, if the department drops a requirement, it may be to your advantage to change your catalog so that you're not required to take that class.
However, if the department drops the class as a requirement, they may also decrease the frequency with which it's offered, so that could mess a student up. In my department, we recently required 222 of all students; previously it was an elective. So for students in older catalogs, we can't require them to take 222, but material that was previously taught in 321 is now being taught in 222, so we tell them it's in their best interest to take 222, even if they're not required to, and most do.
We constantly have trouble with students who assume that prerequisites are just a plot to keep them from graduating early. It doesn't make sense to them that they can't take Practice 1, 2 and 3 all in the same semester- it'll get them to graduation faster! Uh, no, it won't, because you will flunk 2 and 3 because you don't know the material taught in 1. And then there's the student who only wants T-TH classes, it doesn't matter what the classes are. So they're a sophomore, but they enroll in senior level classes because they're taught on T-TH, instead of taking all the sophomore-level classes before moving on to the junior-level classes. Yes, it would be delightful if we could offer all classes on T-TH, but you know what? Faculty aren't machines that can give 4 lectures a day, and there are only so many classrooms on campus. We can't have the classrooms sitting empty 3 days a week because you don't want to come to class MWF.