Actually, no - it would be very difficult to do without ruining the ring's symmetry - and value. If neither of those matter to you, a very, very good local jeweler should be able to do it.
Sizing down would require removing some metal, ruining the spacing of the diamonds, as VRBeauty said. I wouldn't do it. Have you thought about sizing beads? They should be fine for a size down and they can be removed later, if you like. If you are near a Cartier store, I would take the ring and ask them their opinion.