I have yet to take my first cruise, so am definitely NOT an expert, but have been reading up on this quite a bit here because I am hoping for a port upgrade.
My understanding is different from above... I agree that in the example above, they'd look at their "magically derived" chart for the price to upgrade from a 9A to a 4, but that the price you paid for your 9A wouldn't factor in at all. So, whether you booked your 9A on the first day, with an On Board Booking discount, or later when the price had gone up, that the price quoted to go from a 9A to a 4 would be the same. Is that what others understand?
Not trying to be argumentative, but really wanting to understand as much as possible.