You can transform an existing map from one coordinate system to another by querying the objects from the attached source drawing into the current drawing.
To transform the coordinate system of a drawing
As the objects are retrieved from the source drawing into the current drawing, they are transformed from the coordinate system of the source drawing to the coordinate system of the current drawing.
Once the objects are in the new drawing, you can detach the source drawing and save the new drawing. The objects are unchanged in the source drawing, but they use the new coordinate system in the new drawing.