Actually, in the "olden days", like around 12 or 14 yrs. ago (when prices sometimes varied a LOT depending upon which agency you booked through), it was often quite hard to impossible to get an "adjustment" after final pymt.
Back then, we used to cruise every yr. w/RCCL during March, along w/our friends. At best we ONCE got a small upgrade when the price dropped, however we didn't realize that is why we got the upgrade until later. Subsequent to that, there was an occasion involving a booking on RCCL when the same T/A would not give our neighbors an adjustment, nor an upgrade, though the price for their cabin category had dropped significantly. The T/A blamed it on RCCL and vice versa.
I always booked figuring after final pymt., nothing could be done. It was so in my experience. There were some posts on these boards to the contrary, but they always seemed to be people who booked the higher end cabins, booked frequently w/a particular agent.
When the adjustments became more easily available, I was thrilled! The past several years I have come to expect an adjustment when prices drop and have received several. Although RCCL used to be our first choice in cruisng and they have wonderful ships, their prices are higher & we have been using other lines more often as a result. Now they are removing the "safety net", so there is less of an advantage to book with them. The differences between the mainstream big ship lines are not THAT different, so . . . .???