Depends.
If you book under a promo and the promo is still valid for the new dates and the room is still available for those dates and that promo, then your new reservation is still under that promo.
But basically, yes, the new amounts are figured when you pay the final bill. The new 1 night deposit is adjusted as needed, up or down. There isn't a credit refunded if it goes down, you just have a credit on the account but if the new room is more, you'll have to pay more of a deposit.
If you'll go online now, and play around with new reservations (using the change reservation option) you'll see the new rates and what happens.
It's really not at all complicated and will likely make complete sense when you see it all for yourself.