AutoCad :: Rotating Working Plane / Flattening Geometry

Is there a way to rotate the plane you're working on on the fly? I have (what is supposed to be) a 2d floor plan with multiple elements that have non-zero z values, a lot of which apparently cannot be exploded, or sliced (after turning them to surfaces), or anything else. I tried to simply "stretch" the non-zero z vertices down/up to the xy plane and the whole object moves up or down instead. (I'm also using a LISP routine by Lee Mac to do the actual flattening after the fact).

So, what I want to do is simply draw in the 2d geometry necessary to project the object onto the xy plane, and then delete the extraneous 3d lines/objects. However, I'm working on an isometric view (to make sure I'm drawing them in at z = 0), and AutoCAD apparently thinks I want to draw "walls" (yz plane) instead of "floors" (xy plane), if that makes any sense.

Is there some button I can press to change the working plane on the fly? Why AutoCAD wouldn't have this sort of functionality when a much simpler program such as Visual can do it (just by hitting TAB you can swap between the xy, xz, and yz planes).

Alternatively, flattening geometry that does not want to be flattened?