HEAT oven to 400°F. Prick potatoes in several places with fork. Bake 1 hour. Reduce oven temperature to 350°F.
CUT off thin slice from top of each potato. Discard tops; scoop out centers, leaving 1/8-inch-thick shells. Mash potatoes. Add dressing, sour cream and chives; mash until fluffy. Spoon into shells; top with VELVEETA. Place in shallow baking dish.
BAKE 20 minutes or until heated through.