Your best bet is to ask the Travel/Cruise Agent whom you booked the cruise with. They will be able to tell you if any upgrades are available and at what cost.
We sailed on the arcadia in march on a panama canal cruise. we had booked an outside cabin but a couple of weeks before we were due to go we rang up the head office at p & o to say we were interested in an upgrade. they rang back a couple of times with silly prices for a balcony cabin which we refused. we then offered them £200 each for the upgrade which they agreed on. so good luck and have a lovely honeymoon.