For a placeholder you can only transfer within 30 days of when you were onboard and made the deposit for the placeholder -- even if it hasn't been "booked" to a specific future sailing yet.
You can modify a placeholder reservation and retain the 10% discount as long as the embark date is within the original 24 months. If you want the Costco cash card, you'll have to crunch the numbers to see if it's worth losing the 10% discount.