AutoCAD 2018 and Civil 3D 2018 Releases - Backwards Compatibility
It used to be that Civil 3D would not be backwards compatible, year on year. From Civil 3D 2013 to Civil 3D 2016, however, customers enjoyed seamless save back and forward between these versions with no loss of functionality.
In the Autodesk 2018 AutoCAD based release two changes occurred:
The AutoCAD drawing 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
Specifically affecting Civil 3D customers, the Civil 3D objects have been updated between Civil 3D 2017 to 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. Same applies for AutoCAD, however for AutoCAD users it's a simple matter of doing a Save As and saving back to the 2013 drawing file format.
For Civil 3d customers, there is no simple way to 'undo' the changes to the Civil 3D objects once saved in Civil 3D 2018. Saving to 2013 format does not alter the format of the Civil 3D entities.
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.
Civil 3D 2017 data is also 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.