Cook potatoes in large pot of boiling water until tender, about 20 minutes. Drain; mash until there are no lumps. Add cream cheese, sour cream, onion powder, salt, pepper and egg whites; blend well. Spray a 9 x 13-inch casserole with nonstick cooking spray. Add potato mixture. Dot with margarine. Cool slightly, cover and refrigerate up to 3 days. Take out of refrigerator 30 minutes before baking. Preheat oven to 350°F Bake, covered, 40 minutes or until steaming hot in center.
Note: Alternately, potatoes can be reheated in a slow-cooker. Take potatoes out of refrigerator about 3 1/2 hours before you plan to serve them. Place in slow-cooker. Dot with margarine or butter. Cook on low heat for 3 hours, stirring once or twice. They can be held an additional 30 minutes or more.