Will your cat have access to the downstairs of the house as well? If the couple moves in at the same time as you, perhaps the cats will have to be kept separate for a while, until your kitty adjusts to his new surroundings. When he feels comfortable in his new home you can introduce him to the other cats.

Expect a few accidents at first, moving is a stressful thing for cats too. Perhaps some of the others might have better ideas, these are a just a few thoughts I had.