Place sweet potato and margarine in a medium-size microwave-safe bowl. Microwave at HIGH 3 minutes or until potato is thoroughly heated; stir well. Add brown sugar and pumpkin pie spice to potato mixture; stir well. Gradually add egg substitute and milk, stirring well. Pour potato mixture into pie crust.
Bake at 400° for 10 minutes. Reduce heat to 350°, and bake 55 additional minutes or until set. Let pie cool on a wire rack.