ABOUT THE CHANGES IN CIVIL 3D 2018
AutoCAD 2018 heralds a change in the drawing file format to a 2018 drawing file (.dwg) format. This is a universal change for all AutoCAD based products and normally occurs every three years (we got 5 years this time). This means that saving to 2013 format is a requirement from 2018 to any earlier format, for AutoCAD and AutoCAD Map 3D users.
For Civil 3D customers, Civil 3D 2018 also changes the Civil 3D objects to 2018 format (they used to do this every single year, up until Civil 3 2014 where they didn't change the Civil 3D objects for 3 years). When you save, the objects are permanently updated, and can only be viewed and edited in Civil 3D 2018.
If you have opened and saved a drawing in Civil 3D 2018, then it cannot be used directly in Civil 3D 2017.
This is a good summation of the impacts: https://knowledge.autodesk.com/support/infrastructure-design-suite/getting-started/caas/simplecontent/content/autocad-autocad-civil-3d-2018-backwards-compatibility-interoperability-previous-releases.html
GOING BACKWARDS IN VERSIONS
The only way to go backwards is to:
Follow this page regarding exporting a drawing file from Civil 3D with the Civil 3D data removed, and to the 2013 drawing file format (you need to be in Civil 3D 2018 to do this) - https://knowledge.autodesk.com/support/autocad-civil-3d/troubleshooting/caas/sfdcarticles/sfdcarticles/Civil-3D-2017-lost-backward-compatibility.html
Type LANDXMOUT and output the surfaces, alignments, profiles, profile views and pipes to LandXML
Type LANDXMIN in Civil 3D 2017 or earlier version and import these objects into the 2017 (or earlier) format drawing file.