there's no distinction between the speakers other than size. i.e. you speakers weren't "2.0" they were just two speakers, and you used to sets to make four speakers. that said, all you need to do is get two additional speakers to "upgrade" your 5.1 to 7.1. most speakers except for maybe the front two, and front middle, are simple satellites so size appropriately, i.e. if you're only putting out 5W of power, don't get 3' speakers for the satellites.
either way, there should be instructions for hooking up your respective 5.1 and 7.1... probably just separate inputs in pairs (front LR, rear LR, etc.).
taks