Changing the actual username, i.e the username that gets automatically sent to file servers, is not straightforward in Home edition at all.

Required for network logons without password prompt. If the username and that user’s password matches what is on the server, then there is no pop-up prompt for username/password.

Renaming in User Accounts only changes the display name.

Start -> run -> “control userpasswords2”
(no quotes)
then choose properties on the user, and change the username there