Since it's probably done automatically, I think they refund back to original payment method. You can then use that to put it towards another cruise.
I received my port charged and taxes back to my CC…. Part of my payment for this was a FCC from last year so I feel right now that everything including the extra money I paid is now in a FCC because I played around with the site today and it’s attached to my Castaway Club number… I would check because if you are on one of the cruises that was modified you might be able to apply your 25% off to an already existing cruise like your HI one….