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

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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