Honestly, asking a ship's officer about "bookings" is like asking that WDW bus driver, his interpretation is subjective and you can't really expect it to have any impact on what
DCL tells you. He read the letter and interpreted it as you did, but if you look at it from DCL's viewpoint they're talking about people booking NEW cruises, this was never intended to give folks that already booked itineraries the opportunity to change that booking and piggy back a discount that was never meant to be applied to pre-exisisting bookings in the first place. That's all I'm saying, your view is you're being cheated, but DCL doesn't see it that way because how this new discount is being offered.