I first attempted the export command, tried several different formats.
No success...
Command: _AecExportToAutoCAD2004
Command not allowed because drawing contains objects from a newer version of
this application.
Not sure what objects are in the drawing that are prompting this. They used Carlson (not sure of version), not a newer Civil 3d. I tried quickselect to see what type of entities are in the drawing, nothing unusual stands out.
I also saved as older DXF file versions and still recieve a notice when opening drawing, see attached image.
I'll keep looking, thanks for the replies.