A complimentary cabin upgrade is a move to a higher cabin category at no extra cost. In price protection, it is the remedy some lines offer when a fare falls after the point where they stop giving money back.
Royal Caribbean and Celebrity both work this way under their Best Price Guarantee. Before final payment, a lower fare for your cabin category is handled by repricing, with a refund of the difference if you have paid in full. After final payment, you can ask for an upgrade instead: if a higher category on your sailing is priced at or below what you paid, the line may move you into it, subject to availability. Celebrity describes the same repricing remedy as still open within 24 hours after final payment, and the upgrade route beyond that.
In practice this means a drop after final payment is not worth money, but it can still be worth a better cabin. It depends on the category above yours falling to your price while cabins in it are still available, so the timing is narrow and nobody at the line will tell you when it happens.
You or your travel agent make the request, and the line decides. See Best Price Guarantee, the Royal Caribbean price drop policy, the Celebrity price drop policy and the guide to price drops after final payment.
