While I understand your concerns, please note that even if we delete the thread, Google will display your old username until its cache is updated. This is beyond our control. It's moot and unproductive.
Currently, on our forum, the thread starter reflects your new username, not the old one. Whatever you were aiming to accomplish on this forum has been completed: The separation of your old username from the said thread.
You should address this issue with Google directly, as they control the search results. Google is the only platform still displaying your old username.
Needless to say: even if you delete the account, Google will still display your old username until its cache is updated
I hope this is clear enough.