If you move to a different cruise that sails within 24 months of when you booked the placeholder and you are before the paid-in-full date for the first cruise you booked, you can still use the placeholder discount.
If you cancel a cruise booked using a placeholder, you will lose the ability to use the discount.
So, if you apply your placeholder to a cruise and decide before the paid-in-full date that you don't want to take that cruise but don't know when you will want to book another cruise, your best option is to move your reservation to a cruise that leaves shortly before your placeholder would expire. You can then move again once you find a new cruise, or cancel before the paid-in-full date for the new cruise and get a full refund of anything you have paid.