My experience is a little different than what is recommended here. I use a TA that specializes in Disney cruises and I had the 2020 Transatlantic I booked without a placeholder. We went on a recent 4 Day and I wanted to reshop the 2020 cruise since the price was holding steady. My TA said to book a placeholder and when I returned they would reshop with the 10% discount applying the $250 placeholder deposit and transfer my current deposit to the new reservation and keep the same stateroom. They even sent a prefilled form to use to get the placeholder(s). Just to make sure, I questioned the process with the onboard rep and she agreed this was the way to do it if I wanted to keep my stateroom. The problem onboard is they have a slow connection with shoreside and she can’t do the cancel/rebook/deposit transfer so she could only book me into a different stateroom as a new reservation and pay full deposit and do the cancellation and wait for a refund. I would have to deal with a room change with my TA and take my chances my original stateroom would still be available. I dropped off the placeholder form and called my TA as soon as we got home. In less than 10 minutes the TA had a new reservation # for me with all the funds applied straight to it and a cancellation # for the original reservation. Same stateroom. Worked slick!