Converting 2-D DXF to 2-D DXF possible?

In AutoCAD, there is really no difference between 2D and 3D. All drawings are in 3d and the dxf files will reflect that. Even if the drawing is drawn "flat" (not using any 3d coordinates), the dxf file will still save 3d information in its database. The values for the 3rd dimension (the "z" direction) of the objects will simply be zero.

In short, not conversion needed.

Not really. To convert 2D to 3D you need to know where the z position is of all x,y points. If you have full orthogonal views it is possible to define the z position of points on say the front elevation. However many 2D drawings do not have this, yet the brain still manages to produce a 3D image from limited information. The brain is a wonderful thing!