- Disable automatic login. (System Preferences --> Accounts)
- Disable File Vault. (System Preferences --> Security)
- Log in to a user that is NOT the user you are changing (10.4).
- For additional security, backup your data!
- Download the ChangeShortName script.
- Launch the ChangeShortName application.
- Select the current short name you want to change.
- Type in the new short name and long name and select 'Restart When Finished'.
In 10.5 through 10.8:
- Enable the root user.
- Log in as root and navigate to the /Users folder.
- Rename the client Home folder (no spaces or special characters).
- Go to System Preferences > Accounts and create a new user with the same short name in step 3.
- Click OK when "A folder in the Users folder already has the name…". Wait.
- Once the dialog box goes away choose Log Out from the Apple menu.
- Log in as the newly created user.
- Verify everything looks good and delete the original user account via the Accounts pane of System Preferences.
- Disable the root user.
This article references two Apple kbase articles:
http://support.apple.com/kb/HT1428 - How to change user short name or home directory name
http://support.apple.com/kb/HT1528 - Enabling and using the "root" user in Mac OS X