Civil 3D 2017 data is not backwards compatible to previous versions.
Here is some feedback from from Dave Simeone, Senior Product Manager – AutoCAD Civil 3D, as to the reasons for not maintaining backward compatibility:
"Civil 3D Object Format - Yes, we made the hard decision to update our objects for Release 2017. This enabled us to properly implement the improvements in the initial release AND also provide us a platform for some things that we're hoping to deliver during the year."
Moving data from 2017 back to previous releases is still achievable, however not directly using Save As on the drawing. The Civil 3D data aspects need to be transferred via other methods, such as LandXML.