I also got my information from a top-selling RCL travel agent. I admit my interpretation may not be perfect, it was how we interpretted it at the time - we are waiting for clarification to come in.
BUt I ask you what you mean when you say "book a cruise" - I checked with RCL and the online gaurantee says when you "make a reservation" defined by RCL as making a deposit.
At that point you have just 48 hours to look for a better price. I stand by only being eligible for an ungrade up until the time final payment is made (if there is a price drop) because the idea there was to eliminate the need to cancel and rebook. That period is only good up until the cancellation penalty period starts (when final payment is due).