You are put into a "pool" for any available balcony cabin. You'll get at least an 8a balcony. Maybe higher. It depends on what others have booked as far as specific cabins is concerned. Early booking won't necessarily help in getting an upgrade....but it definately is "factored in" when decisions, if necessary, have to be made whether to upgrade one person nstead of another.
We booked a balcony gty on Carnival Pride, and at first, when we first found out our stateroom, we were placed on a high deck. Then, when the prices went down for out cruise before we sailed, I called them and they gave me a $100 shipboard credit for the difference. After that, they immediately moved us to the lowest 8a cat. We ended up upgrading to an extended balcony on a higher deck after that and just paying the difference.
Moral of the story....Don't ask for a credit when the prices go down unless you want to be dumped to the bottom of the barrell.