I have recently been asked by people, who have opened Civil 3D 2006 drawings in 2007, what can they do to save the data back?
Well, because of the dwg change and thus a change in the definition of objects in Civil 3D 2007 (as compared to earlier versions), it is not possible to simply "save as" to an earlier version and access the intelligent civil data.
So the question is, what options do we have?
TS1056107 offers some guidance.
In order to VIEW drawings, in earlier versions of Civil 3D or in AutoCAD, as proxy graphics (see note below), you will need to use the AutoCAD Save As command.
To EDIT Civil 3D 2007 drawings in earlier versions it is necessary to "Export to AutoCAD" which creates a drawing with all of the intelligent civil data exploded to AutoCAD entities.
TS1056337 mentions that if we "export to AutoCAD while in plan view, the objects will not retain any elevation information.
To retain elevation information we can either explode intelligent civil objects twice to turn them into AutoCAD entities or use the "export to AutoCAD" while in a non-top view.
Lastly, please do not forget that we have the ability to Export to a LandXML file the Civil 3D Data and Import somewhere else. This works for most Civil data even though Civil 3D 2006 uses LandXML 1.0 and Civil 3D 2007 uses LandXML 1.1.
Note: Some users have been experiencing problems with their older drawings when opened in 2007. It has been recommended that they set PROXYGRAPHICS to "0".