Open the dwg file with Rhinoceros, hack away at it, and save it as a dwg file. Rhino can do all that you would logically expect AutoCAD to do with surfaces, and more.
Better, open a _copy_ of just the parts you want to change, and edit that. Rhino may not preserve some AutoCAD entities.
It may be necessary to add one linear dimesion in Rhino to avoid a scaling error.
Mike Halloran
Pembroke Pines, FL, USA