I think the only addition I have (which likely isn’t the scenario asked about) is that if you used an OBB for a cruise that is cancelled by Disney, they can apply that OBB benefit to an already booked cruise (without adjusting for prevailing rates).
We had our Feb 2021 cruise cancelled by Disney, and they applied the OBB I used to my upcoming Oct 2021 cruise (which I had already booked). No change in original booking numbers.
That being said, I have a strong suspicion that my Oct 1 sailing in the Wonder won’t sail. If Disney cancels it, I don’t know if the policy of applying the OBB forward is still in effect. I hope it is.