Step 1: Set up the SPN for the user accounts You have to set the Service Principal Name (SPN) for the farm account on the computer that is running SharePoint Server 2007. To do this, you must have the Setspn.exe tool from the Windows Server 2003 Service...

To change the user account you can use STSADM migrateuser option and change user name in the MySettings pages for that user. stsadm.exe -o migrateuser -oldlogin < Domain name \ Old user name > -newlogin < Domain name \ New user name > ...