When saving a DGN as a DWG, Smart Solids converts into Mesh. In some cases, it might be needed to save Smart Solids as Solids when saving in DWG format.
1. Open the DGN
2. Go to File > Save As and change 'Save as type' to DWG
3. Click on the Options button to open the Save As DWG/DXF Options dialog
4. Select the General tab and expand the option Entity Mapping
5. Change the setting for Flat Solids and Surfaces Surfaces from Polyface Mesh to ACIS Entity (Body or 3D Solid)