The native DWG drawing file format for AutoCAD 2010 is not compatible with previous AutoCAD releases. AutoCAD 2010 can open drawing file formats from previous releases dating back to the 1980’s with AutoCAD 2.x, In order to open AutoCAD 2010 files in previous releases, you will need to use the SAVEAS command and save it to the appropriate DWG file format or set your default Save format version in the Options.
Below are the list of drawing file formats that AutoCAD 2010 can save to, and the AutoCAD releases covered for that DWG format version:
ObjectARX API Compatibility
AutoCAD 2010 breaks binary compatibility with previous releases. ARX applications will need to be recompiled and perhaps modified to run on AutoCAD 2010. API binary compatibility is for compiled ObjectARX and does not affect LISP and script customization.