I did try and mash pre-baked potatoes. Don't bother, it's not worth it. You might as well boil the potatoes. I had to cut it up and simmer it with milk to get it warm enough to mash properly.
Also, I suggested to Nick that he freeze some leftover sour cream that we had. Don't do that either. It breaks down and separates. You can still use it and it still tastes okay, but it's thin and a little lumpy.