A. Open your list of skills ('K') and click on the trade skill you wish to unlearn. At the bottom will be a description of the skill. Next to that is an icon that you can click to unlearn the skill. Be warned, this will completely remove any knowledge you have of that skill, and therefore if you wish to relearn it in the future, you will have to start from the very beginning.
Yes, you can change them, but you will start at level 1! Make sure you really want this new profession and you a willing to get rid of all your past work.