Are you attempting to change the "short name" of your user account? Yes, you must enable Root to do that.
Follow these instructions with great care:
Be sure to disable Root when you are finished.
About the account name
Each user in Mac OS X has a "Full Name" (referred to as "Name" in Mac OS X v10.5.8 or earlier) and an "Account name" (referred to as "Short Name" in Mac OS X v10.5.8 or earlier) as defined in the Users & Groups pane of System Preferences (referred to as Accounts pane or Users pane in Mac OS X v10.6.8 or earlier). The account name can contain up to 255 lowercase characters with no spaces. This is the name used to create a user's home directory (also known as the user's Home or Home folder) in the Users folder. Once the user has been created, the account name (or short name) cannot be changed in the Users & Groups pane of System Preferences.
Changing the account name
Though there are other methods by which an advanced user may change the short name and related information, the easier and safer workaround is described below.
Before following these steps, you may wish to back up your important data.
For Mac OS X v10.5 or later
- Enable the root user.
- Log in as root.
- Navigate to the /Users folder.
- Select the Home folder with the short name you want to change, and rename it just like you would rename any folder. Keep in mind that the shortname must be all lowercase, with no spaces, and only contain letters.
- Use the Users & Groups pane (Accounts pane in Mac OS X v10.6.8 or earlier) in System Preferences to create a new user with the Account name or Short Name that you used in the previous step.
- Click OK when "A folder in the Users folder already has the name 'account name'. Would you like to use that folder as the Home folder for this user account?" appears. Note: This will correct the ownership of all files in the Home folder, and avoid permissions issues with the contents.
- Choose Log Out from the Apple menu.
- Log in as the newly created user. You should be able to access all of your original files (on the desktop, in Documents, and in the other folders of this Home).
- After verifying that your data is as expected, you can delete the original user account via the Users & Groups pane (Accounts pane in Mac OS X v10.6.8 or earlier).
- Disable the root user.