The only line that has different rules for paid in full booking vs deposit booking for lowering rate prior to penalty (at least the only on I'm aware of) is HAL. The rest will simply credit your card for the difference of the drop.
As you mentioned, RCI and Celeb will typically drop your price even inside penalty and Carnival will as well. The one day sale promotions are usually the only promos that they exclude from being dropped on existing bookings. It's always worth a call to find out though.