Actually, once you reach a certain number of posts (I was thinking it was around 1000 to 1250), you can change it by going to "My Controls" at the top of the page and clicking "Edit Profile Information" on the left. When your profile info comes up, it will be the first item, called "Custom member Title".
I've heard different things. Some say they've been able to change them at 1100 and others that it was after 1300 but I'm not sure. It probably depends on which software version was in use at that time.