Sub Demo()
Application.ScreenUpdating = False
Dim Rng As Range
With ActiveDocument
  Do While .Footnotes.Count > 0
    With .Footnotes(1)
      Set Rng = .Reference
      With Rng
        .Collapse wdCollapseStart
        .Text = "[!!]"
        .Start = .Start + 2
        .Collapse wdCollapseStart
      End With
      Rng.FormattedText = .Range.FormattedText
      .Delete
    End With
  Loop
End With
Application.ScreenUpdating = True
End Sub