TL:DR; Batch convert DWG and DXF files back and forth between R14, 2002, 2004 and 2007 versions of DWG used by AutoCAD.
Download OverCAD DWG DXF Converter Shareware - Rating: 4.73
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Batch convert DWG and DXF files back and forth between R14, 2002, 2004 and 2007 versions of DWG used by AutoCAD.