I had a query recently about how to convert digitised plots of land from one coordinate system to another in Civil 3D. This is similar to a previous post, so here is a different way of doing the same thing effectively with possibly less steps.
First open the drawing with the plots to be converted. Set the current coordinate system in the drawing by using the MAPCSASSIGN command. Then export the polygons to SDF(or SHP or whatever you wish) file format by typing MAPEXPORT. Choose the polygons/layers that you wish to export. On the Options tab you can specify a coordinate conversion as part of the export, see below:
Now in a new drawing (with or without the second coordinate system set), type MAPIMPORT and select the file exported previously and it will import your plots to the new coordinates.