I have owned both - Started with an Impy and now shoot an Ion... The Threads are the same on both for the feedneck and the barrels, so those are interchangable. Also, the ASA (where your air bottle screws in) is the same fitting. Otherwise, I think everything's different. You certainly cannot use an impulse board, tapeworm, any internals, eyes, etc.
My personal recomendation for ups on the Ion would be:
Smartparts Q-Lock Feedneck - Its low rise so it'll stop the hopper being above your head and it'll hold it nice and tight.
ASA with an ON/OFF - I wouldn't use a drop forward tho unless you have little arms. It adds to the hight of your marker otherwise. I use an Eclipse on/off which takes about an inch off the overall length of your setup.
Barrel Kit. - Look on the forums - Probably a Freak Kit, as you can match the inserts to any Paint you're shooting. Probably a 14'' front (Thats the overall length of the barrel)
I wouldn't touch the Reg- Its kinda awkward to use but i've been told it actually re-charges (15%) faster than the Cp reg and the other aftermarket ones. The NEW Shocker comes with the ION reg now - in a sleave. Thats how good it is. Just let it settle by shooting a couple of rounds after adjusting but before you chrono.
Change the trigger if you want - but the stock's adjustable. not really a priority.make sure you have a fast hopper tho - either a Halo B or Reloader B. Egg's seem to have had their day and aren't true force feeders.
Hope this helps
Scooby