You get 10% off the total fare and you are required to pay a 10% deposit.
If you move your cruise, you will continue to get a 10% off the new cruise fare (in your example, it will be $700). You will also need to maintain a 10% deposit, so you will have to pay the balance (in your case $300).
Note that fares are based on the time you book or later move your cruise, if you move it to a new date/itinerary. This could result in higher per person rates by moving your cruise.
If you have a date in mind, best is to book it directly. If you cannot afford the deposit at the time, book the cheapest category. You can then later move to a higher category and pay the difference, but you will maintain the original booking day prices.
If the date you have is further out the published dates, then book a dummy cruise (or how
DCL calls it - a placeholder) for the furthest possible cruise. Once new dates are announce, you will then move it right away and get the best possible rates - opening date rates.