Book the Sept. 2024 now. Then when Sept. rolls around check the pricing to see if it's gone up and how much. If the OBB discount would still be beneficial - buy the placeholder when you are onboard in Sept. 2023. Once you are home you can then call to use the placeholder for the Sept. 2024 cruise -- note that the discount will be off the prevailing rates (when you make the change) not the opening day rate. But if the prices have increased enough to negate the discount, no worries as you've gotten the best price by booking opening day.