If somebody rents a field from his friend and agrees to pay him a fixed payment, part of their agreement is that he will plant barley in that field. If he agrees to that, the renter would not be allowed to plant wheat in the field, because wheat takes more out of the soil and weakens the land more than barley does. So the agreement was that he would focus on barley and plant barley. He can't switch and rationalize, “Oh, I'm renting the land, so I can do whatever I want with it.” No, if you agreed on a crop that's going to be less burdensome on the land, you have to stick to it. You can't switch to, for example, wheat, which is tougher on the land. Chittim yizra'enah se'orim. But if you agreed to plant wheat in that field, you're allowed to switch to barley because barley puts less strain on the land, so therefore we would assume that the owner has no problem with it.
Rabban Shimon ben Gamliel says that even if you agreed to plant wheat, you're not allowed to switch to barley. Even though barley is less tough on the land, the owner may not want you to switch back into barley from one type of crop to a different type of crop, because when you switch year to year, it weakens the soil. Any type of change would weaken the soil. So therefore, Rashbag says, any change is a problem.
Similar halakha (Jewish law). If the renter agreed that he was going to plant grain in a rented field, he's not allowed to switch and plant beans instead, because beans weaken the land more than grain does. Kitnit yizra'enah tevu'ah. But if you agreed to plant beans over there, you're allowed to switch to grain, because grain strains the land less. Rabban Shimon ben Gamliel says, “Not allowed. Not allowed to make any change.” Same thing as we said before: any change would strain the field and weaken the land year to year. You don't want to take all the nutrients out of the soil. By the time the soil adjusts to one type of crop, you don't want to switch it to something else. So therefore, any change whatsoever—whatever is different from the agreement you had—would be a problem according to Rabban Shimon ben Gamliel.