When I replaced my HW heater I looked at furnace assy and thought this isn't too bad to remove but still, there is the wiring, fuel, exhaust, and etc. I didn't see the aquastates when I opened the front panel looking at the burner stuff, so yeh, I reckon it will have to come out.
Personally, if I were doing it, I'd look at pulling the HW heater and the board it sets on as well. Not that it's necessary it's just that I don't bend as well as I used to and it would really open up the enclosure. FYI, removing the HW heater takes about 15 -20 minutes.
Alternativily, I would simply pay someone, set back with a beverage, and learn for the next time.