Convert3dm is a simple commmand-line utility that reads a 3dm file and writes a new 3dm file using the specified file version.
Convert3dm support Rhino file versions 1 through 7.
Convert3dm works on both Windows and Mac, and it does not require Rhino to be installed.
Convert3dm is free!
Download the version of Convert3dm appropriate for your operating system. Then unzip the file and copy the program into a folder that is in your system's search path. Then open a command prompt or terminal window and run the Convert3dm command.