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.