I think I'd start now, only to help establish the habit beforehand, since you're the Mama Dog, so to speak.
While it's fine for them to realize that new baby is higher on the totem pole than they are, it's good to have the least negative association with new baby as possible. So starting any changes beforehand in terms of where they're allowed to be, etc., would be good I think.
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Another tip I remembered:
Get a nightlight lamp -- one of those cheapo lamps (Hobby Lobby has lots usually), that takes a nightlight bulb. If you can't find one, then get a nightlight wattage bulb (they sell them at Wal-Mart and Lowes) that will fit in a regular lamp. Put the lamp high up (like on a shelf or bookcase) near the changing area. That will give you enough light to change baby's diaper in the middle of the night, but won't require you to turn on a bright light or risk waking baby more fully.
You can get blackout curtains/shades, or just hang a heavy blanket over the window, too.