i'm p sure you need to make 100(?) posts before you're no longer a "junior member"
but you can change it yourself through the custom user title option on the my profile tab on the settings page http://www.belltreeforums.com/profile.php?do=editprofile
Say you do change it, I think you have to change it again though, after you do the 100 posts and become Senior Member.
After 100 is Member.
200 is Senior member.
Say you do change it, I think you have to change it again though, after you do the 100 posts and become Senior Member.