What will happen to anew user's account if he wishes to change his username after some months in the forum?
nothing, if they want to change their username they can either donate to the forum or request a name change from Theymos. to be honest if they want to change their username but their account is still newbie rank, it would be much better to just create a new account.
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?
achieving a high rank doesn't they'll be able to change their username as was mentioned before, they can either donate or for a name change from Theymos, and no, changing their username doesn't mean they will be suspected of changing hands.