Straight from the terms @boze9999 posted: Per Stateroom (not per person), must resubmit for verification each year, and new bookings only.
If you have credit sitting in your account, you can call HAL and ask them to apply it to a booking - I've done that. When I've called in for something, I've also had them say "you have $xx OBC available, do you want me to apply it?"