The subject behavior occurs because the User Accounts applet in Control Panel only changes the full name of the account, NOT the logon name.

To change the logon name:

1. Start / Run / compmgmt.msc / OK.

2. Expand System Tools.

3. Expand Local Users and Groups.

4. Press Users.

5. Right-click the account that you wish to rename and press Rename.

6. Type the new name and press Enter.

NOTE: if you have Scheduled Tasks that use the old logon name, or system services, you must adjust them.