First, this is why you should always book with an agent so you can let them take care of all this for you and you don't have to hassle with it or worry about it. They will also be able to tell you if the lower rates that are connected to any specials you may or may not qualify for. Plus, agents don't charge anything for their service and have alot of information and advice to offer, so it makes sense to utilize them whenever possible. And if you book it directly with the cruise line and have any problems, who do you think they represent? It's always good to have someone on your side.
But in this particular case, since final payment has not yet been made, it should not be a problem getting your fare reduced, provided the reduction is not a special for 'new bookings only', in which case they may give you a hard time about. But this far out, I doubt that would be a problem.
Always keep your eyes open because you never know when a special will reduce your fare. Then jump on it right away because it could be a very limited special.