Sub FixTableBorders()
Application.ScreenUpdating = False
Application.DisplayAlerts = False
Dim Tbl As Table, Bdr As Border
With ActiveDocument
  For Each Tbl In .Tables
    With Tbl
      For Each Bdr In .Borders
        With Bdr
          If .LineStyle = wdLineStyleSingle Then
            Select Case .LineWidth
              'Case 2 To 6: .LineWidth = 8
              Case wdLineWidth025pt, wdLineWidth050pt, wdLineWidth075pt: .LineWidth = wdLineWidth100pt
            End Select
          End If
        End With
      Next
    End With
  Next
End With
Application.DisplayAlerts = True
Application.ScreenUpdating = True
End Sub