I'm using a recipe for make ahead mashed potatoes with sour cream and chives. So far so good. But it calls for bread crumbs, but doesn't specify fresh or dry! You sprinkle the crumbs over the top of mashed potatoes before you bake. Thanks.
If you're baking the potatoes for a long time, I'd be inclined to use fresh bread crumbs, but I wouldn't put them on until you're about to pop the dish into the oven.
Some recipes I've looked at do call for crumbs made from dry bread.
Since the mashed potato is already done and you are simply re-heating and crisping, I would sprinkle with panko and a drizzle of butter. Heat until golden on top.