It's just E all the time now
I think the reasoning behind this, is because Pendolino's have standard carriages A-D, akin to Super Voyagers, where as First Class differs between the two classes, which is why Pendolino's now have coach E unreserved all the time.
This means that if a [5-car] Super Voyager substitutes a Pendolino, all standard pax should still have reserved seats.
If a Pendolino substitutes a Super Voyager, not only will standard pax get their seats, First Class pax will have plenty of seats to choose from compared to normal.