Você pode colocar esse código no objeto ThisDocument
no VBA.
Private WithEvents App As Word.Application
Private Sub Document_Open()
Set App = Word.Application
End Sub
Private Sub App_DocumentBeforeSave(ByVal Doc As Document, SaveAsUI As Boolean, Cancel As Boolean)
ActiveDocument.Fields.Update
End Sub
OU - se você estiver usando o comando SaveAs:
Sub FileSaveAs()
Dialogs(wdDialogFileSaveAs).Show
'returns the name including the .doc extension
ChosenFileNameAndExtension = ActiveDocument.Name 'Or use .FullName
ActiveDocument.Fields.Update
End Sub