Toast the pecans at 350°F for 6 to 8 minutes until fragrant and slightly darker. Remove from the oven and cool completely before chopping.
Whisk together the mayonnaise, Greek yogurt, Dijon mustard, lemon juice, salt, and pepper in a small bowl until smooth. The dressing should be thick enough to mound briefly on a spoon.
Add the cold chicken, grapes, celery, red onion, parsley, and dill to a large bowl and toss once or twice to combine.
Add about three-quarters of the dressing and fold gently until the chicken is coated. Add more dressing only if the mixture looks dry.
Fold in the cooled pecans just until distributed.
Cover and refrigerate for 20 to 30 minutes before serving. Stir gently and taste before serving; adjust with lemon or salt if needed.