Sometimes, they offer the next cruise, which would be a b2b, at a reduced rate. Now that ships are running full, it doesn’t happen as often. However, it did happen to us just last month, because the next cruise was not fully booked. The guests OB, jumped on that and filled up the ship.
I checked prices on the future ones, that I was interested in, and none were a good deal. IMO. Just check with them early on. YMMV