i believe you are misunderstanding the way 'most' 3d modeling is current done. In most systems (Solidworks, Autocad 3d, proengineer, blender, sketchup etc...) you do not create a 3 view in 2d of the object to create 3d one.
Normally, you have to 'build' the a part by using methods such as extruding (either by distance, along a path, in an array, etc...), revolving, subtracting, intersecting. When you get the individual parts completed, you 'join' them to create the model. They are usually created just as they would be in real life then mated just as they would be in real life as well...
So to answer the question in your comment, in solidworks for example, lets say you wanted to create a 2'x2'x4' 'box'. You'd draw a 2'x2' rectange, then extrude it up 4'. You'd have your box.
If you wanted to change that to a 4' cylinder w/ diameter 7', you WOULD be able to 'edit' the original drawn box to be a circle w/ a diameter of 7' and in this simple example, the resulting 3d model would in fact change to a 4' cylinder w/ a dia of 7'.
Hope this helps...