hi agian brneyes, Carnival's website doesn't have a spot that I'm aware of that you can check online for such promotions as 55+ or military rates. You have to call them to find out. They typically ask during the booking to make sure they get you the best rate code category but don't make it easy to see if one opens later.
On the subject of before or after final payment drops... You should always try to get the lower rate if there is one available at any time. The reason for the distinction being important is not the rate but the rate code you're booked in. If the rate code you book in falls in price you can get it adjusted at any time, at least with CCL up to 24-48 hours before the sailing (depending on when they finalize the manifest or sell out your category). Many time new rate codes appear for last minute bookings or rate codes that apply to different categories of cabin. These circumstances change things. Sometimes they will still allow you to change to the new promotion/category (most cruiselines will not). Occasionally they will have an asterisk stating "*For new bookings only". Carnival is one of the best at not excluding you, even once in penalty and often in other Cats and promos. You simply have to ask you T/A or the cruiseline (if you booked direct) if you can be moved to the new promotion. I've had very good success with having all of this done for my clients in the past year or better with CCL. They are very accommodating with wanting you to have the lower rate and keep you happy and coming back. I wish that were the case with all lines but it's the exception, not the rule.