The short name is also used as the name of users home folder so changing it is not as easy as just changing a field in a dialog. It is a good idea, though, if the user has inherited the Mac.

1. If Automatic Login is enabled (in Accounts preferences) for the account you’re modifying, disable it. Similarly, if File Vault is enabled (in Security preferences), disable it. You can re-enable these features, if desired, after completing the procedure.
2. Log in as a different user than the one you want to modify; make sure the account you want to modify is not logged in.
3. Open the Accounts pane of System Preferences.
4. If the lock icon in the lower-left of the Accounts window is “locked,” click on it and provide an administrative username and password; this allows you to make changes.
5. In the list of accounts on the left, right-click (or Control-click) on the name of the account you want to modify; choose Advanced Options from the resulting menu.
6. In the Advanced Options screen, delete your current short username in the Short Name field, and then type in your desired new short username.
7. In the Home Directory field, change /Users/oldusername to /Users/newusername, where oldusername is your original short username and newusername is your new short username. Make note of the original and new paths.
8. Click on OK and close System Preferences.
9. Open Terminal (in /Applications/Utilities).
10. Type the following command, all on one line, and then press Return; when prompted, provide the password of the admin account you’re currently using, and then press Return again:

sudo mv /Users/oldusername /Users/newusername

These are the original and new Home Directory paths from Step 7; oldusername is your original short username and newusername is your new short username. This step renames your home folder in the Finder.

After you restart the Mac all will be fine.

The largest problem this will cause is if the user is using Time Machine to back up their Mac. Since the folder name has changed Time Machine will re-back up the users entire home folder.

There is also a known problem when using personal web sharing.

If you do discover an Application that doesn't work please tell me so I can find a fix/work around and add it to the instructions, thanks.