HEAT oven to 350°F. Melt butter in medium saucepan on medium heat. Whisk in flour; cook 2 min., stirring constantly. Gradually stir in milk; cook until mixture boils and thickens, stirring constantly. Add Velveeta; cook 3 min. or until melted, stirring frequently. Stir in macaroni.
SPOON into 2-L casserole sprayed with cooking spray; sprinkle with Cheddar and cracker crumbs. Cover.
BAKE 20 min. or until heated through.