Just check the web sites for lower fares (this site, icruise.com, www.mytravelco.com
, cruise411.com, etc etc.) Even Check RCCLs site. Once you find a lower price, call your TA. Your TA has to handle all price changes. BTW, some of the cruise lines now don't honor all "lower price" finds after the initial booking. This is due to regional discounts, last minute deals, age-based discounts, etc. It's getting complicated, and I no longer feel that it is cheaper to book way in advance. However, if you are outside of the cruise lines cancellation date for a full refund (usually 60-70 days), you can cancel your reservation and rebook to get the lower fare (you TA may charge you for the change though).
In the articles section on this site is a good write up on this (Kuki or ErnieMC wrote it).
By the way, if you are looking to save some bucks on a cruise, check out icruise.com's Visa card. You can save $500, $800, or $1000 off of a cruise once you hit certain spending thresholds. They return about 3.5 cents for every dollar you spend on the card.
Best affinity card I've seen. RCCL also has a credit card, but it doesn't seem to be as good a deal as Icruise