That's complicated. The issue of retrofitting is easier with more recent equipment. I think Mr. Parsons addressed that issue. Some coaches that were built, say, in the last 10 years already have the structural integrity to take belted seats. They already have the rails in them. However, if you're looking at a much older coach, one that's near the end of its lifespan, a 15-, 16- or 18-year-old coach, it probably doesn't have the structural integrity. It would be worse to slap down a couple of strips of nylon and staple gun them onto the seats. That would be worse than not putting anything on. So, whether retrofitting is viable or not depends on the coach and the age of the coach.
On April 9th, 2019. See this statement in context.