Fall 2018 will most likely be announced in April/May. If they are announced before you sail, book and reshop it once on board. If they have not been announced, book a placeholder when you are on the ship and move it on opening day.
When you cruise, check the price, if the current price - the 10% discount is less than what you paid then you will need to cancel your existing cruise and book the new one. If the price has gone up more than the discount then you already have the best price.
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