"Well met (character name). I am Senji. I serve the Oracle of the Mists, by finding individuals the proper occupation based on their abilities. Do you wish to broaden your horizons and see if another path is right for you? For a one-time fee of 500 gold I can teach you a new secondary profession. I can also return you to a previous secondary profession free of charge."
On switching to a new secondary profession, you will lose access to all the skills of your previous secondary profession, and will be given a basic set of skills from the newly selected secondary profession:
You will need to pay the 500 fee the first time you change to a specific profession with Senji, even if you have completed the corresponding profession-switching quest in the Prophecies campaign. For this reason you can save money and switch in the Crystal Desert instead if you have completed the appropriate quest.