How does one lose one's junior status?
I don't run the forums, so I'm only guessing here...
If you look at the Members List and sort by number of posts, all the Senior Members have over 100 posts. All the Members have over 30 posts. All the Junior Members have less than 30 posts. So it looks like you need to post your way out of junior-ness, but please don't post just for the sake of getting rid of the word 'Junior' in your title!