Since this was linked to in a recent thread, I'll throw in that in my experience you get pretty much complete backwards/forwards compatibility. Altium can import CS files without issue, and CS can open AD files without issue provided you save them (from AD) as CS format files without issue, including the 3D models. I did this a couple times when I was evaluating CS.
I've just had a look at AD again and don't see that option but I've also uninstalled CS, the exporter may have been packaged with CS.