The first two times we sailed on the Magic (2010 and 2011) we booked an inside no-view room and were upgraded to an ocean view both times. We were upgraded for free when they assigned our room, before we even got to port.
The last time we were on the Magic 2013 we tried the same "trick" without success. So we planned to upgrade at the port. We were literally the first people at the check-in desk and there were no, zero, upgrades. Bummer, so we decided to try the inside no-view room. Boy are they small. I went up to customer service (?) and they said that there were no rooms, but to wait until the ship left port and check back. They put our name on a waitlist. After we left port they called and said that they had a verandah room available that we could have for extra $$, but I forget what the amount was. We took the room and they took our luggage to the new room. The funny thing was that they called and apologized for any problem we may of had and left a bottle of wine in the new room for any inconvenience.
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I think that they always have some rooms that are unoccupied. Something might go wrong in one room and they need to put you in a different room, example: major repairs or maintenance in the room that you were assigned.
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