OS X Mountain Lion: If you can’t change the account name
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You can’t change the account name for a user after the user has been added. If you need to change the account name, you must delete the user and create a new user with the new account name.
Important: If you delete a user account, all the files and settings in the user’s home folder are deleted unless you select one of the options to save the home folder.
Last Modified: Sep 4, 2015