Speak to P&O - RCI/Celebrity honour price drops FYI so use that as a lever.
If you used an agent get stuck into them you may not get your money back but could get a good upgrade it is not in the agents best interest to have unhappy customers best of luck?
No you won't be able to do anything. the price you booked at is the price to pay. its the same as a supermarket/shop. if you bought a dress one week, then saw it was on sale a couple of weeks later with 30% off, you wouldnt go back to the shop and ask for the difference because you have already bought the dress. unfortunately this is just the way it is.