I wondered about this shortly after the 500 came out. The way to do this, IMO, is to swap the ring/pinions...NOT the whole diff. The front 500 diff is missing the extension housing on the right where the diff lock mechanism lives (making you wonder why the 500 doesn't have a diff lock, but that's another post).
The other issue is the lack or a rear wheel speed sensor hole on the 500. Which leads me to a question for
@HondaTech...
Digging through the parts diagrams the 700 an 500 share multiple parts in the rear diff:
Left side half of differential
Inner axle cup (which means the ring has the same splines)
Ring bearings
Output coupler/seals
The output bearing shows to be a different part number, bu the reference numbers in the descriptions show them to be the same size?
Seems to me you could potentially swap the ring/pinion for different gearing. I suppose they wont just drop in as a full assembly do to difference in mounts and potentially size.
I'll look at the front and see whats up there as well.
These part similarities are all referencing 2020 models of both the 500 and 700.
...where is the tone ring for the rear speed sensor on the 700? Made into the ring and pinions? If so, I doubt it's there on the 500, since it doesn't use it, which kills the whole thing.
Also, suppose we get it to work. How does the ecm react to higher rpm vs speed? Does it flip out and go into limp? Deluxe guys can simply go full manual I suspect...but is that why you bought an auto? (Keep it down 500 guys...)
These are all reasons I suspect this has never been done thus far. Back when I 1st wondered about this, it was going to be like $1200 in parts for something that had no guarantee of working. Now you can probably find them cheap enough used to take the risk...if you're a hardcore mud guy.
the guy never got back to me on it. but hes on portals and diffs. so the speed would be way off to begin with. i know he had the diff being drilled but that was about the only picture
. im thinking you could get a true speed reading but the rev limiter would hit in 3rd gear. cause at the lower speeds i dont think youd be able to romp on it but then again im not sure how the shift points are aligned is it off speed vs rpm or only speed and throttle input. cause i know i can romp on mine and itll shift out to 2nd at like 15 or 18 give or take or ease on it and shift around 8-10 mph. thats enough to make me think that if the 500 diffs did throw it off i still think the auto would shift right but you probably dont want to hold it to thw floor till its in 3rd gear. there is alot to think about in it. but hopefully by the end of this yr ill have all the answers for it. 
