I believe member titles are automated, and they depend on how many posts you have- you changed from member to veteran member because you reached a new plateau of posts (300, in your case)

Here is the list of titles, and the post numbers that reflect them:
- Newly Joined - 0
- New Member - 1 post
- Junior Member - 6 posts
- Member - 20
- Veteran Member - 300
- Grand Member - 500
- Poohbah - 1000
- Grand Poohbah - 1500
- Magnate - 2000
- Grand Magnate - 3000
- Elder - 5000
- Wise Elder - 7500
- Legendary - 10000
- Legendary Wise Elder - 20000
- Wisest Elder Ever - 50000