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vanoman,

Попробуйте вариант, предложенный здесь.
И взгляните на смену logon name - How to change your logon name (user account name) in Windows 8.1 and Windows 8:

Windows creates a logon name based on the first name and your full name is stored as the display name. You can easily change your display name from the User Accounts Control Panel but what about the logon name? You can change the logon name too without having to create a new user account but the way to change it isn't so obvious. Here is how to do it.

В случае отсутствия результата после выполнения инструкций, изложенных в двух ссылках, уточните, где возникает старое имя?? Лучше покажите срином.