The Mishnah in Kelayim 2:3 records that if one wishes to plant barley in a field currently seeded with wheat, or vice versa, he must wait until the field is wormy, plow it, and then he may seed it. If the wheat has sprouted, he must uproot it before he can seed the barley.
In Mishnah 4, it addresses the same case, but with grapes instead of barley. In the various permutations thereof, the Mishnah rules that one must uproot/plow over what is there, and only then may he seed. However, if the grapes have sprouted already, he may chop them down to within a tefach of the ground, plant the wheat, and then uproot the grapes.
Why is this not true in the other cases? If my wheat has sprouted, why can't I chop it down to within a tefach, plant the barley or grapes, and then uproot the wheat?