[Logician]

Do everything that threads say, except change the line:

PHP Code:

if ($bbuserinfo['usergroupid']==2 AND $bbuserinfo['joindate']>strtotime("-3 months") ) 


AS:

PHP Code:

if ($bbuserinfo['usergroupid']==2 AND $bbuserinfo['posts']>2000 ) 


This will make sure your users usergroup will change automatically when they reach 2000 posts.

This is the easiest solution for your request. I think that it is easier to change your other sections of your site than changing default forum structure of vbulletin.