Susan, Carnival has been much more lenient about doing this even past the actual due date recently. Never pay your final payment until due. You could be earning interest on the money!
Even if you have paid the entire balance before it was due they should do the adjustment up until the due date. Just because it is paid doesn't mean there can not be adjustments made during this time.
Another thing to keep in mind though is if you booked when there was a lower fuel supplement, they will most likely include that in the new rate.
This ate up some of my first OBC with Carnival. Since you are no where near the official final payment due date, I would call them back and get the adjustment. Even if there is a fuel supplement adjustment, and it eats up some of the credit or OBC. They will at least have everything straightened out for possible future adjustments.
I just went through this with Carnival last month. My first OBC, took quite a while on hold, while they figured out the new fuel supplement calculations. I was locked in for $5, and the new was $9. The next 2 went much easier after.
I am not a HAL expert though. Hope this helps.