Has the old version got the underfloor structure to change the seat round
That's certainly a part of it, but there is more, and more important: For rear facing seats, the seat[back] itself will be required to support the crash loads prescribed for the plane in it's certification basis. This will be at a minimum 9G and a factor of safety of 1.33 or 1.5. It may be as high as 18G x 1.5. That means that the seat back will have to support at least 2035 pounds, and maybe as much as 4590 pounds. Thereafter, when I approve such installations, I assume that the plane may still crash with a force exceeding the crash load requirement. In such a case, I assume the failure of the seat back, and the occupant striking whatever is next in the path of destruction. I want to assure that nothing in the seat back would be injurious if it collapsed or penetrated the seat back. So, no bracing the seat from it's back side - the bracing could buckle, and become a penetrating danger to the occupant. I really like to see rear facing seats which are up against a robust cabin bulkhead. Larger airplanes which have floor attach seats may be qualified for either direction by the approval of the seat itself, but this is uncommon for light GA planes.
I've done this testing and broken seats in the process. I've also passed a number of seat installations, but it's stressful watching the seat and its attachments deflecting under the test load!