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?
No, the request to change your name from your initial name to a new name, the requirements that you submit are official on this forum, you have to pass several requirements to fulfill the request for a name change, maybe some of the members above have already said that.
Here, many account names have been changed to new ones, as far as I know, all of them are legal, in fact some of them have received praise for the new name, it's not what you think (accusations), if you want to change your account name, First you have to reach the Legendary position first and have the best contribution in this forum. If you only know about Bounty, don't expect your new name to be granted.
So try to be the best and top poster here, so that you can change your current name to a new name, for example changing money to gold and so on.