Changing the units isn't going to change the elements. You need to scale the objects in the drawing by a factor of 1/12 (0.083333333). You can do this either before or after you make the surface.
Except that I think he said the objects are incorrect in their elevations only. Scaling will mess up the x and y.
If that's the case, here's a lisp routine that will change only the elevations of your objects. This will select all LWpolylines not on frozen layers, so freeze all layers except the layers that contain your contours/polylines.
Use Appload to load the routine. It will run automatically upon load.