This method use Revit's IFC import/export functionality.
Basically create an IFC of the current Revit version, then import into previous version of Revit.
The resultant previous version Revit file will NOT contain properly made Revit objects, but may be sufficient as context information.
Read the full method here:
http://whatrevitwants.blogspot.com.au/2014/06/using-revit-2015-for-quick-high.html