The bottom half will definitely shoot again (probably several shoots), but likely will not flower for a couple of years. The top will be fine. It's better to do this in spring though (aka after this set of flowers has faded).I can't shorten it by potting deeper. Since it produces some new roots at intervals all the way up, I suppose that I might have to nerve myself and cut it in two - with lots of roots above the cut, and hope that the bottom half will shoot again for a second plant.