What will happen to anew user's account if he wishes to change his username after some months in the forum?
There is no guarantee admin would approve account name change for any new members, unless there is a very good reason for that.
If the user has achieved a high rank in the forum, and choses to change his username, will the account be suspected of changing hand and be tagged for that? If yes how will other users know he is still the rightful owner?
It makes no difference if username is changed for high ranked accounts.
Everything is archived and ID number of account is staying the same even if visible name is changed.
I wouldn't worry and waste time on things like this.