The new 120-300mm is only a cosmetic and software upgrade. Guy from Sigma at Photokina states this fact. It is a cosmetic and software upgrade as all new sigma lenses will be able to be connected to a dock to "modify" their settings. One example he gives is that the AF can be modified to be faster or slower with compromises at each end.
Sigma USB Dock First Look
I'm not surprised, the design of that lens was pretty new and made little sense to modify it. If any, it sure had more mechanical than optical problems - I think to some extent deriving from the heavy weight of the lenses. If they managed to shrink it due to this new composite materials, then probably all the better for the AF motor and our backs.
Reading the specs though I think the 17-70mm was modified. We shall see what Sigma intends to do with the other lenses. A new version of the 24-70 f/2.8 HSM would be most welcome. That lens is amazing, but the bokeh is nauseating - strange enough, a feature in which Sigma often performs outstandingly.