I always assumed for signs it would be a case of stickers (like they do already for other changes) and then replacement over time as signs got renewed. I guess to go all out, we might need to move exit markers slightly given they're spaced 100 yards apart I think, but they're close enough to 100m for it to probably be fine

I think most cars for as long as I remember can do both

Changing the test just seems like a bit of bureaucracy, but maybe I'm simplifying that in my head