To the best of my knowledge, you should be OK if the replacement tires are of the same circumfrence. The tractor is designed to provide for somewhat more "ground covering" by the front tires when you are in four wheel drive (so that steering is not adversely affected). In Kubota speak, the ratio between the front and rear is "gear ratio". I learned this when I asked Kubota for some drive train information. You might be OK if the new rears have a smaller circumference, but I can't quantify how much smaller is too much smaller. Please refer to the attached. I may be giving you information you already have, but better more info than less.
Good luck, and let us know how you like the tires.