here it is in a nutshell ---
First - the old policy is completely gone unless you booked your cruise before May 17. In that case you are still fully entitled to the old best price guarantee.
With new bookings it works like this:
If you put down a deposit only before the final payment is due you can watch for price drops and put down the final payment whenever you want at the lower price. You are NOT obligated to pay the price the cruise was when you made your deposit. But you must make the final payment before the penalty period starts (usually 90 days ahead of the sail date) or else pay whatever the price is on the day final payments are due.
If you happen to have made a full payment and the penalty period has not yet started and the prices drop you can ask for an upgraded stateroom selling for the amount you paid for your stateroom (assuming all the cruise fares on the ship went down) and RCL will try to accomodate you.
In the absense of these two conditions (meaning you booked during the penalty phase by paying full price) you are stuck with the 48-hour best price guarantee limitation. If you see a better price within 48 hours you fill in a form showing the lower price and you will receive a 110% shipboard credit for the difference between what you paid and the new price. This offer ends 48 hours after you book your cruise by making the reservation and leaving a deposit.