If for example you change wall openings in a wall. Just delete the wall in C4D, then export just the new wall. It should drop into place in the C4D model, as long as you have not moved or changed the scale of the model in C4D.
I have added elements in the same manner. There is an eXchange plug-in available that allows more control over this process.
http://www.maxon.net/pages/connections/archicad/archicad_e.html
