I once tried to transfer a reservation made through
DCL to Costco, who gave us a 10% rebate on the trip. I WAS able to do it, but not without a problem. A DCL supervisor called me and told me that if I did this, that I would not own the reservation and that Costco could now "do anything" to it that they wanted. I asked why a
travel agent might change my reservation without telling me, and she conceeded that they wouldn't if they wanted to remain in business. But she really made me insecure about doing this. In the end, there were no problems, but the cruise line was not happy about it, probably because they had to pay Costco a commission, which they didn't have to do when I booked directly through them.