Can I change the spacing or days to maturity of just one planting?

Yes, so a special case never throws off the rest of your plan. When you add or edit a planting in the Game Plan, Heirloom fills in its days to maturity, spacing, time in the nursery, yield and price from your Charts; change any of them in the planting window and the change stays on that planting only.

Your charts do not move, and neither do your other plantings. To change the starting value for the next plantings of the crop, change it in the chart instead.

One case works the other way: when a crop has no chart yet for that planting method, Heirloom offers to save the values you entered as its chart when you click Create.

The steps are in Add a planting and Change your crop and cultivar values.