Actually, there is a real way to change this username.
For lazy people :
To set the current user's short name:
$ defaults write /Library/Preferences/com.apple.NetworkAuthorization UseDefaultName -bool NO
$ defaults write /Library/Preferences/com.apple.NetworkAuthorization UseShortName -bool YES
To set a custom name:
$ defaults write /Library/Preferences/com.apple.NetworkAuthorization UseDefaultName -bool YES
$ defaults write /Library/Preferences/com.apple.NetworkAuthorization DefaultName "user"
Replace "user" with the desired custom name and enclose it in quotation marks.
To set no name:
$ defaults write /Library/Preferences/com.apple.NetworkAuthorization UseDefaultName -bool YES
$ defaults write /Library/Preferences/com.apple.NetworkAuthorization DefaultName ""
To set the current user's long name:
This is only necessary if you have made any of the changes listed above.
$ defaults write /Library/Preferences/com.apple.NetworkAuthorization UseDefaultName -bool NO
$ defaults write /Library/Preferences/com.apple.NetworkAuthorization UseShortName -bool NO
or
$ defaults delete /Library/Preferences/com.apple.NetworkAuthorization UseDefaultName
$ defaults delete /Library/Preferences/com.apple.NetworkAuthorization UseShortName