Soory to jump in so late. IMHO, the function should return the error code only in EXIT @ERROR and the $ChangeVLKey whatever the SET/GETOBJCT generates (0 in case of success otherwise WMI error code, thus
Code:

Function ChangeVLKey($VOL_PROD_KEY)
  Dim $obj, $objects
  ;Key is without hyphens (ABCDEFGHIJKLMNOPQRSTUVWXY) 
  $objects = GetObject("winmgmts:{impersonationLevel=impersonate}").InstancesOf("win32_WindowsProductActivation")
  If @ERROR
    ;ChangeVLKey should not have a value if we got an error 
    $ChangeVLKey = Val('&' + Right(DecToHex(@ERROR), 4))
    ;Convert the error to readable format 
    Exit Val('&' + Right(DecToHex(@ERROR), 4))
  EndIf
  For Each $obj in $objects
    $ChangeVLKey = $obj.SetProductKey($VOL_PROD_KEY)
  Next
  ;Assumes we got here okay so exit with no error 
  Exit @ERROR
EndFunction
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