From what I have found

The CAAD9's rear triangle is different. The seat stays go into the seatpost a little bit wider than on the CAAD8. Also all of the tube intersections/joints have been widened and smoothed. Apparently a large difference where the top tube and head tube meet improving front end stiffness. I have also heard the tubing is butted more on the CAAD9 saving a couple of more grams (~25g).

All fairly small changes that generally improve the frame.

I own a CAAD8 and have no complaints with it.