I think the Magic situation is interesting here. They cancelled all the original summer itineraries and are doing short 2-4 night UK resident only cruises instead. I could see them doing the same thing for the Dream/Fantasy/Wonder the first few months of sailing: cancel everything, create a whole new slate of itineraries, and have everyone rebook--ideally with some sort of preferential access for guests that had been cancelled.
They did something similar with restaurants at WDW last summer. All of our restuarnat reservations were cancelled all at once, but we were the first to be able rebook dining for our August trip.
Alternatively, they could cancel everyone in some categories (e.g., inside/oceanview) and given them an option to rebook to a different cruise in a verandah at no cost. That would be similar to what they did with the hotels at WDW last year: they closed a lot of the moderate/value hotels and rebooked those guests in a deluxe at no cost.