Yes, if you have OS X 10.5 (Leopard), it's pretty easy. Go to System Preferences, then Accounts. In the list of users on the left, ctrl-click (or right-click) on the account you want to change, and click "Advanced Options". You'll see a panel where you can change the short name of the account.
1. Create a new user account. This does not require reformatting or reinstalling.
2. If you are using Leopard, go to System Preferences/Account right click on the user you want to modify and click Advanced Options. For this to work you want to change your home folder to the name you want then change it under Advanced Options. Change the short name then select the new home directory. They must match exactly.
There is a Freeware program called ChangeShortName 1.3 that will allow you to change the short name in systems 10.3x and 10.4x. Go to VersionTracker.com and search for "Change Short Name" ...
Karen is absolutely correct. You cannot change a short name in Tiger. I copy and pasted this from my Tiger Help Viewer:
"You can't change the short name for a user account after the account has been created. If you need to change the short name, you must delete the account and create a new user account with a new short name."
The app she suggested at Version Tracker should do the trick and I am grateful that she had that information as I wasn't aware it was available!