The deal is much better if you re-book while on board. There's really no danger in it--you can change your dates or cancel entirely up to 60 days before your scheduled sailing. If you cancel, you get a full refund. If you re-schedule, the same deal is applied to the new dates. Currently, the special is 10% off plus $200 stateroom credit on a 7 day cruise.
In years past, you had 90 days after returning home to call
DCL and get the special. However, they now put Donna (their TA) on the ship, and you must book while on the ship.
In my experience, Donna is not the sharpest tack in the box, and doesn't always offer the best deals. Your best bet is to know exactly what you want and take an internet print out of the deal you want. I've seen rumors where she matched a print out. For me, she twice denied that the deal existed (2 separate cruises.) The first time, I went home and booked. This past time, I booked thru Donna, then called DCL when I got home. They allowed me to "switch" to the internet deal and still get the additional 10% off plus the stateroom credit.
Ok, sorry....I shouldn't have maligned Donna. She's a nice lady. She just doesn't go out of her way to find a deal for you...she punches her computer and tells you the first thing that comes up as if it's the only price you'll find.