I hate to be a stickler, but did they actually 'agree' to this rule, meaning was the transfer ability in writing in the cruise contract?
If it is then they definitely needed to have notified you and honored what was agreed when you made your reservation.
if not, then your reservation was not directly affected, your cruise and everything that comes with it are the same. The credit is through Costco and has nothing to do with Disney. So unfortunately it was a gamble to wait it out 7 months after your reservation was made assuming nothing would change. Since it's nothing that directly affects your reservation, while I understand the frustration, I don't feel it was negligence on Disney's part, or a scam, etc,for them to not send out a general email to everyone who has a reservation just in case they may or may not want to transfer it somewhere along the line, even if your intention was explained when you booked it. Because at that time you would have been able to do it.
Companies change their policies all the time, so if the credit was that much of a priority definitely would've booked it that way from the start and worried about the room placement issue after. If the credit was through Disney and they all of a sudden taken away, then once again definitely that would be an issue. But it's in a gray area where nothing between you and Disney has changed, just the way to deal with third-parties.
If it IS in writing somewhere, then definitely fight it! Sorry this has dampened things for you. And don't mean to stir things up, just giving another point of view. Hope you still go and have a wonderful cruise!