Book as early as possible. That is your best bet to get the lowest fare. Rarely, if ever, do the prices go down as we get closer and closer to the sailing date. They almost always go higher as the ship starts to fill up. If your cruise is in the latter half of May, then it will probably fill up. The kids will be out of school in the South and high season will be kicking into gear.
Unfortunately, May of 2016 is relatively close in terms of cruise scheduling. I'd be surprised if any discounts came out at all, much less at this stage of the game. So, again, book now before fares get higher.
In the unlikely event that a new discount is offered, you probably won't be allowed to apply it to your existing reservation. Your only other option is to do the math and see if it saves you money to cancel your first reservation and rebook a new res with the discount. Again, not likely, but keep an eye out just in case.
Finally, if you have flexibility in your room location and category, check out the IGT/OGT/VGT sticky at the top of this board for a way to save a good chunk of $$$.