I can't directly answer your questions, but -- the new insert piece is required for babies between 4lbs and 5lbs, and then optional after that (at least on the Canadian version I've seen). Required if your baby needs it to boost them up so their shoulders are at the lowest harness slot. It comes in two pieces that velcro together at the lower back area, and the upper body padding can be used for longer.
I doubt that it could be used in older SR35s, but the only way to know would be to phone and ask. Report back so anybody asking in the future will know?