Actually I didnt get the goal?
If I didnt get you wrong, you ask a link in User CP which is visible to ALL users and your users' usergroup will change automatically when they enter their "forum user names" there. It seems that everyforum member will be able to do it as everyone has a "forum user name" so what's the aim to do in the first place? You can just put all users in that special usergroup when they registered, or better, make your registered usergroup "special" by assigning the relevant permissions.
The hack would make sense if you wanted *some* of the registered users change their usergroup automatically. Eg. some are provided a password and by entering this password their usergroup would change. (like in vb.com). If this is what you want this algorithm here can help while you are building yours:
https://vborg.vbsupport.ru/showthrea...179#post301179