First, an agent should NEVER move a client without checking with them first. The only time this would may be acceptable is if the agent and the client had a long term relationship and the agent knew the likes and dislikes of the client.
Unfortunately, there are some very unscrupulous agents out there. One of the many little 'tricks' they like to pull on unsuspecting clients is the old, "Guess what? I just got you a free upgrade!" What many newbies don't understand is that the price went down and instead of the agent calling the client and notifying them of this fact, the move them up to another category so they don't lose any commission and then have the audacity to tell the agent they got them a free upgrade.
This is why I always tell people to keep watching the rate on their cruise - sometimes you may see something the agent simply doesn't see (they can't watch everything for everyone). If the price changes, notify the agent immediately. And if the agent ever calls you and says you got a free upgrade, check it out - it may be legitimate, but it may also not be what they want you to believe.
Bottom line; most agents are very hardworking and reputable - they work very hard for their clients and have their best interest at heart. But it's also important to make sure you know exactly what you're getting, the price your paying, never hesitate to ask for written confirmation, and always double check everything. After all, it's your money.