If I change a cultivar’s days to maturity, do my existing plantings change?

No: once in your plan, a planting keeps its own days to maturity, so a change never shifts your season behind your back.

What new plantings start from is the Charts. In Direct seeding or Transplantation, change the Average DTM on the cultivar’s row, or on the crop’s row for all its cultivars: the plantings you add from then on take that value.

To bring plantings already planned in line, click the three dots of the chart row and choose Apply to existing plantings, or change a few at once with Multi-edit in the Game Plan.

The steps are in Change your crop and cultivar values and Update the plantings already in your plan.