You haven't mentioned the type of room you have booked. I assume that your agent booked you in a stateroom for 3 or 4 guests. Depending on the type of stateroom, the 3 & 4th guest will either sleep in bunk beds attached to the wall or sofa beds.
In general, if you booked through a travel agent, NCL prefers that you make any reservation changes through the travel agent. However, if you have an issue with the agent, you can explain it to them and they should be able to help you.
As far as switching the room is concerned, check out the types of rooms available and based on the stateroom type you are interested, you will have to pay the difference for the upgrade.
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