re: Buttermilk vs milk or cream

I don't think it will work, Darlene.
Commercial buttermilk is always either low-fat or non-fat. It will curdle when heated to high temperatures. I've had curdling problems when using even sweet low-fat milk in baked potato casseroles. I have since been advised by a home economist that some butterfat is needed (in the form of either heavy cream, whole milk, half-and-half, or full-fat evap milk) to keep the white sauce from separating and curdling in the oven.