Dry the grapes thoroughly after washing. Wet grapes are the main reason this salad turns watery.
Add the brown sugar and pecans only when the bowl goes to the table, so the topping stays crunchy.
Make-ahead: build the salad through the chilling step up to 24 hours in advance and keep the topping in a separate container.
Storage: refrigerate leftovers, covered, for up to 3 days. Do not freeze, because the grapes soften and the dairy separates.
Substitutions: plain Greek yogurt for the sour cream, walnuts or toasted pepitas for the pecans, or crushed graham crackers for a nut-free crunch.
For small children, halve the grapes lengthwise and pat them dry again before folding them in.
Nutrition values are an estimate.