Sub DemoA()
Dim Cmnt As Comment
For Each Cmnt In ActiveDocument.Comments
  Cmnt.Author = "Internal"
Next
End Sub

Sub DemoB()
Application.ScreenUpdating = False
Dim DocSrc As Document, DocTgt As Document
Dim i As Long, j As Long
Set DocSrc = ActiveDocument
With Dialogs(wdDialogFileOpen)
  If .Display Then
    Set DocTgt = Documents.Open(FileName:=.Name, _
      AddToRecentFiles:=False, Visible:=True)
  End If
End With
If DocTgt Is Nothing Then Exit Sub
With DocTgt
  For j = i To .Comments.Count
    .Comments(j).Author = "External"
  Next
End With
With DocSrc
  For i = 1 To .Comments.Count
    With DocTgt
      For j = i To .Comments.Count
        With .Comments(j)
          If .Scope.Text = DocSrc.Comments(i).Scope.Text Then
            If .Range.Text = DocSrc.Comments(i).Range.Text Then
              .Author = "Internal"
              Exit For
            End If
          End If
        End With
      Next
    End With
  Next
End With
'cleanup
Set DocTgt = Nothing: Set DocSrc = Nothing
Application.ScreenUpdating = True
End Sub