To be correct here. You don't really know if the price of your cruise dropped, it's just you have found other travel agents who are offering it for less.
This is not the same as the cruise line reducing the price of the cruise.
It's fairly simple... If you found a lower price you can mention it to the TA you booked with, and see if they'll match the price. If they aren't able to, you're far enough out that you can cancel, and then book with a new agent.
Though, you may want to be sure there were no cancellation fees agreed to with the first agency.