I'm a fellow user. If you want to change your forum display name (username), it should also change it in backwards fashion to prior posts as well.
To do that, click the user icon in the top right corner of this page and select My Settings.
The My Settings page may already open with your forum username displayed, and you simply type the new one in the box.
If it doesn't open that page then navigate My Settings > Personal tab > Username subtab.
