Why can’t I update existing plantings from a cultivar row?

Because that cultivar has no values of its own yet: it uses its crop’s row, so Apply to existing plantings is greyed out, and your plantings still get the update from the crop row.

Apply from the crop row instead: it reaches every planting of the crop in the season you pick. Or give the cultivar its own values first, and the option works on its row.

The steps are in Update the plantings already in your plan.