I was due to get on a 2x5 IET▸ the other day and before arrival it was announced that first class was in coaches 1,2, 6 & 7. The external coach displays were also showing numbers instead of letters. I wonder if this is a general change from letters to numbers for IET coaches. It could certainly be less confusing, with coach 1 always being at the front of the train.
Surely there will need to be both. The numbers are in a fixed order along the platform, and can be (and apparently are being) painted on it. When the train arrives, its carriages will be identified by letters as usual, according to what they are and to locate reservations. If you want to find a reservation, you need to know the mapping from letters to numbers for that service. For that, being told which four carriages (by number along the platform) hold first class is not enough - you need to locate one, and if that fails you certainly want the right unit.
Now, how you get told that remains to be seen. I seem to recall that for
TGVs▸ , at some bigger stations, there are screens showing you this so you know where to stand.
GWR▸ did a while ago have posters, but they are a bit less dynamic than they need to be.