Most primary professions have capabilities that other characters need.
Every character can learn any or all of the secondary professions. All of your progress in the forgotten profession will be lost † and your decision will not be undone if you change your mind, so be sure you want to change. If you want to learn a different primary profession after you have learned two, you can unlearn one or both of the ones you know. Every character can have up to two primary professions at a time. Professions are either primary professions or secondary professions.