As Thor said, it all depends on which "parts" your looking at. Generally the only difference between the 2011-Current "Value Series" Sportsman 400 / 500 / 800 are the Engines and Decals. Chasis/Frame, Transmission, AWD Diffs/Components and Body Plastics are all the exact same (which makes them easy to build and label the "Value Series").
So it your looking at "bolt on accessories"..... items like bumpers, lock & ride, winch/winch mounts should be the same!!!
But if your looking for "performance upgrades"..... they would be unique to the engine size of the machine, and it would be easier to trade up to a larger cc.