Is it still possible to 'buy up' to the next cabin on one segment of a booking?

I recall previously you could do some kind of surcharge of something around 400 GBP per sector from economy to business --- anyone know whether this still is in place, or what the $$ amounts are these days with the advent of premium economy?

It was occaisionally useful if you only had to be in a meeting/awake and aware after one leg of the journey.