Here is another production utility to convert drawing files to the different dwg and dxf versions.
You can download it from here.Teigha File Converter is a free cross-platform application provided by the ODA to end users only for the conversion of .dwg and .dxf files to/from different versions. The application features a graphical interface and a command-line interface, and accepts the following inputs:
Input file filter such as *.dwg (default is "*.dwg;*.dxf")
Upon execution, the application identifies all files in the specified source directory which match the specified filter, then loads and saves each of these files to the specfied output directory as the specified output version/type. If the audit flag is enabled, an audit/repair operation will be applied to each file as it is loaded. If the recurse flag is enabled, subdirectories within the source directory will be processed recursively.