Those sizes sound more like
R4 industrial tires rather then A1 ag tires. But to answer your question the first number is the diameter of the tire, the second number is the width of the tread and the last number, is the diameter of the rim. Putting a wider tire on 9.5 instead of 8.5 may cause interference with your tractor when turning sharply. I would check with your Kubota dealer to see what he would recommend. I don't believe ag tires where an option on a BX200. It think there was just turfs or industrials, but I may be wrong. Usually when switching to ag tires, it is also necessary to change out your rims since ag tires are not as wide as turf or industrials.