Impala, Caprice, Biscayne, Bel Aire, 150, 210; all the cars are the same only the trim is different. The body of each car is either a two door or a four door, and the roof line is either a hard top or sedan (has a post in the middle that frames the front door window), and of course the bubble top which was a hard top with a different "C" pillar (rear quarter).
If you don't like your current trim level you can change it (if you have a source of replacement parts). You can make an "Imp" a "Biscut" or turn a base model into a fully optioned Caprice. All you need is a bank account stuffed with cash and a source of parts. A lot of the SS cars were also only a trim level having no affect on the car with the Ss package being only trim pieces rather than suspension and power train upgrades.