As ever, WoWWiki has the answer.
Wowwiki wrote:
You may unlearn a profession and start a new one but this removes the chosen profession. If you were to learn it again, you would have to start leveling it from a skill level of 1 again. You will also forget any recipes you may have acquired in your old profession so they must be reacquired if you take it up again. The new profession you choose to replace it with also starts with a skill level of 1. You can unlearn a profession from your skills tab (the hotkey is k). To do so, click on the appropriate profession, and in the bottom part of the panel is a tiny icon that when moused over will tell you it lets you unlearn your profession. Be sure you really want to unlearn a profession; Blizzard will not undo it if you change your mind!