In less than five years, the entire tuition and financial aid system at the nation’s top colleges has been overhauled. Today, it looks a lot like a highly progressive tax code, in which the affluent are bearing an enormous share of the overall tuition burden. No matter how high the published cost--almost $50,000, typically--these top universities have become significantly more affordable for the majority of students. The changes have not been greeted with a chorus of joy, however. At colleges without large endowments to subsidize tuition, administrators are worried about the pressure they will face.