HI, I book thru my PVP all the time and know the drill way too well. If your rate has dropped and you are past final payment... pretend to book and see what category now matches what you paid.
I just call up my booking agent or the toll free number and they change me. I think it helps if you already know your facts and what you want when you call.
I dont think it would make any different who you booked with since unless you booked early saver rate, you cant get the price difference once you are past final payment. I have never had trouble getting a nice upgrade though.. but do look at the prices and the deck plans and figure out where you want to be and ask for that cabin.
I am assuming you did not book Early Saver rate, or they would have not told you that you cant get the price drop. .. but you can upgrade.