You are dealing with two completely different things. Seat "comfort" which is excellent all through the various amenity levels - think second class is equivalent to business class on a plane and anything north of that is just puffery - but some people like puffery. Italo trains have leather seating all the way down to second class by the way. They are all comfortable, regardless.
The other issue is the flexibility - the ability to change your mind. As said above there are three levels of flexibility and they are all available - initially anyway until they sell out, cheapest first of course - at all amenity levels.
Me? Cheapest, second class if I am paying. First Class fully flexible if somebody else is. YMMV