will you be reheating stovetop, oven, or microwave?
if you have a choice, reheat in the oven, and prep so that you have a layer of breadcrumbs tossed with olive oil & black pepper and/ or a shredded cheese topping, then you have mashed potato casserole and you don't have to worry if the edges get crispy because that's how it should be
(but also yes to the extra milk and/or butter)