You may as well let them see the preliminary packing. If they see things going into boxes, they seem to precieve that as redecorating; if everything disappears suddenly, well, I'd panic too. Get as much unpacked as you can before they're moved, so familiar objects and smells will be there for them. If they have a particular thing that's the center of their safe universe (like Tigger's bed), make sure it's in place before the cat arrives. Now, move the cat box, move the cats, let them know where the box is. Have drinking water ready for them , and food if they're not panicky, otherwise it'll get ignored or thrown up. Once they know they're safe, and that things are settling into a routine again, things will be fine. This is one of those situations where the cats know they aren't in charge, so if you're calm and as efficient as you can in these circumstances, they'll think you are in charge for once, and feel better about it, because they trust you.