You can only have two primary professions at a time. But they are not 'cast in concrete', you can discard a primary profession and replace it. And they are dirt cheap - Apprentice level training in a primary profession costs 9File:Copper.png in your starting area. When you are first starting, you might benefit by switching professions to meet a goal. You can use this to get a fair upgrade to your starting gear cheaply, and stock up on some low level consumables.