There seems to hardly be any way to have your cake and to eat it too - I mean retaining some of the benefits of the credibility carried over from the old name to the new name, so of course, there can be some benefits in changing your user name, but seemingly the ONLY way to completely lose the connection of the possible negatives that have come to your old user name would be to create a new account and to really start over.. and maybe not even mention the old user name in connection with the new account.
There is also security through obscurity, too.. and there are likely other means of attempting to NOT draw the wrong kinds of attention to yourself.. and the interwebs are filled with fucktwats..
This would be something theymos could create: a new rank that shows you're an old user, without disclosing your old account.
If Admin only creates such accounts for users who made a name for themselves, it could be a solution to have and eat the cake!
I too wouldn't mind having more privacy, but once it's gone, there's no way to get it back.
I am not sure if your proposal is really fair for other members, so a lot of this remains a kind of balance in which many of us might come to differing conclusions. Of course, if we are in a BAD situation, we might want to have as many benefits as we can get, but if we are NOT in the situation, we might blame the other user(s) for possibly contributing to his/her own problem by employing weak OpSec. So, yeah, there are degrees, and sometimes even the ability of members to go back and wipe all of their previous posts can be a cost upon the membership to have to suffer with members who seem to NOT take responsibility for their earlier posts.. but then again, 4 or more years later, there might be some motivations for members to delete some prior disclosures.. and sometimes even to edit earlier posts to change what had actually been said and the context.
Surely, I am NOT proclaiming to know any of the answers exactly, even for myself, because even I understand that I am likely NOT the same person who I was nearly 6 years ago when I began posting in this forum. Also, sometimes I might disagree with some kind of content of the post of another person, or even some kind of mixing of real world and forum situations, or become sympathetic to something that seems to have had happened to another member in the real world that might be attributable to forum information/conduct, but then the circumstances are NOT always well known or even knowable either. I have thought one way about the circumstances of another forum member, and then a third forum member comes into the scene to assert another scenario that I had not considered in my prior assessment.
When I first joined the forum, I, actually, was quite bothered by forum practices that allowed editing or deleting of prior posts, but now, I understand that there can be some value to members who might get themselves into a bit of a pickle (and might not even have had been of their own creation) and might need to edit or delete some or all of their earlier posts without asking for permission from forum administrators.. even while the forum itself may end up suffering somewhat from the removal of some of those earlier contributions and context for other posts that might have been built around their earlier posts.
I'm curious if anyone would even notice. 
The other guy is a full member who seemed more recently active but I guess he's not that active *shrug* a username is a username, just wouldn't want anyone attempting an impersonation on you later on and people getting confused. One can hope people aren't that stupid but.. well.. people have fallen for these things before. The guy seems legitimate enough not to do that though. 
