If I apply a chart from the crop row, are cultivars with their own values updated too?

Yes, so one click brings a whole crop in line. Applied from the crop row, Apply to existing plantings reaches every planting of the crop in the season you choose, including cultivars that have their own values in the chart.

To update only one variety, apply from its cultivar row: only that cultivar’s plantings change. Either way, Heirloom tells you how many plantings will change before you click Apply.

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