The User name and Account name fields are not synchronized (Sync) in MOSS when the user account information is changed in Active Directory (AD).

 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> -ignoresidhistory