I'm shortly taking delivery of a new bike (to me anyway) fitted with SRAM rival compact chainset (50/34). I want to change it to something like 52/39 which is fitted to my current bike with a compact spider. The BB is new so doesn't need changing. My question is can I do this by buying New chainrings, or will I need a new chainset. I'm trying to avoid spending excess money as I'm after New wheels soon anyway.