Travel agents sometimes book a group of cabins and can sell them at a reduced price and if you are lucky to be going at the same time, you can get in on the deal. Booking directly with a cruiseline will not get you the group price.
I think if the price does go down, you would have to cancel and re-book and I don't think there would be much savings at all, if any. Sometimes it will cost you more and you would not have the cabin you origionally booked, so be happy with what you have. Usually too, the t/a will throw an onboard credit to each cabin, or something special that you would not get by booking direct.