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