One important point to keep in mind is that "land" varies a lot, and "pasture" can mean anything from dense grassy pasture with >1 cow per acre to arid pasture

One important point to keep in mind is that "land" varies a lot, and "pasture" can mean anything from dense grassy pasture with >1 cow per acre to arid pasture >100. They aren't convertible to cropland in the same way. Even in terms of "rewinding," one acre doesn't equal another, for most interpretations of "wilderness value." This is why the amazon-2-pastureland problem is so serious. Every acre of amazon rainforest reoresents a lot of habitat.