Renaming or Recommisioning?
Commissioning is the act of putting a boat into service. It may include a collection of parts of the boat to be reassembled and working right, prudent maintenance functions, and a certain amount of aesthetics. It can be pretty labor intensive when a boat is new or has been out of commission for a period of time. If you are paying to commission a boat it can be quite expensive but that depends on the specifics of the boat and the nature of what is involved in recommisioning her.
Renaming a boat costs as little as the elbow grease to remove the old name, the supplies for the ceremony to satisfy the seas, and the materials to put the new name on. Of course if you elect to pay someone to do all of that it can be expensive but usually it is not too bad.