I have not tried to change yet but I contacted support and they said the biggest the rims will fit is 4" so if you want bigger treads you will need to buy new rims still thinking of asking my lbs what they think
So after doing some research you should be fine with up to 4.6-4.8 tires was looking at some charts for the rim size of 85mm and all say good up to 5" but says you should go in the middle for a better fit
Chain could very well be an issue though didn't even think of that issue lol I purchased a set of studded cake eaters that are 26x4 120 TPI. So I will wait for next summer to try anything else hopefully one of you try before me so I know for sure lol but frame and chain are a good distance away from tire I will have to check the clearance at the chain when in "granny gear"
Ultra FS tire size I think depends which year. Older ones may only clear 4", but they might have updated 2020 model for bigger tire clearance.
If it were me, and I wanted to fit the biggest fattest snow tires, I'd do it on the Ultra 1000 (the hard-tail). more clearance when not dealing with rear suspension. Most serious fatbike snow-riders run hard tail frames for a variety of reasons, and tire clearance is one of those reasons.
I have the Ultra 1000 Hard tail and put Maxxis Minion 4.8" on it. I believe the rims that come with the bike are 80mm and with that size rim the tires measure about 4.375" wide. To get 4.8" I think you have to put the on 100mm rims. Anyway, the MM 4.8" tires have much larger knobs on them and look a lot bigger. Very happy with them! This doesn't answer the question about the J 1000 FS though.