No, it is not student->student or regular->regular. If you qualify for the academic pricing you only need to buy the academic version once. From that point on, paid upgrades are the same price. There is no special pricing for an upgrade from an academic license to a regular license.
I purchased Lightroom 4 as an academic version. The price was $79 at the bookstore on campus. When Lightroom 5 was released, I purchased the upgrade (NOT an academic version) online from Adobe for $79.
Purchase Lightroom initially at the best price you can depending on your situation and what you qualify for. From that point forward you can always upgrade at the standard upgrade price.