It could perhaps be a) put to other uses or b) no longer grown in such large quantities and replaced by some other less harmful plan (speaking about corn in particular which is heavily subsidized and terrible for the soil it grows in).

Is that a problem or a good thing. We currently suffer from deforistation due to agriculture. If those feilds could be reforested that would be a good thing. Of course pure capitalism doesn't allow for that, but with a little government intervention (aka, buy up the unneeded feilds) or economic decay (feilds become abandoned due to their lack of value ;) ) then reforestation could occure.

Or there could be solar panels put there, or the corn could end up producing the input to the vat. Or hemp could be grown there which could be used as a construction material, or ...

Processed into feedstock for the meat-vats. Roughly the same workflow as before, just streamlined.

Even bioreactors that produce pharmaceutical products are running on carbon-based energy derived from agricultural output -- glucose gotta come from somewhere. Fixed nitrogen, amines, gotta come from somewhere.