Toss grapes with sugar, lemon juice, and cinnamon in a wide 6- to 8-quart heavy pot. Bring to a boil, covered, over moderate heat, stirring occasionally. Continue to boil, covered, stirring occasionally, until grapes begin to burst, 20 to 30 minutes.
Transfer grapes with a slotted spoon to a colander set over a bowl and drain 15 minutes, then return juices in bowl to pot.
Preheat oven to 425°F. Boil juices, uncovered, until syrupy and reduced to about 2 cups, 20 to 25 minutes.
Stir together Cognac and cornstarch until smooth and whisk into grape syrup. Boil, whisking constantly, 1 minute. Stir grapes into syrup, then spoon filling into a 3-quart shallow baking or gratin dish.
Pulse together flour, brown sugar, oats, butter, cinnamon, and salt in a food processor until mixture resembles coarse meal. Transfer to a bowl and stir in pecans.
Sprinkle topping evenly over filling and bake in middle of oven until topping is golden brown and filling is bubbling, about 20 minutes.
