Thanks for your response...
I understand the concept of changing between pressures. I also understand that certain operators have different procedures, with some changing only on passing the TA and others changing as soon as they are cleared to a FL/altitude.
However, I'm interested to know if anyone is aware of any departures or arrivals which specifically state a point at which to change to QNH/1013?
In my mind, this would be particularly useful if a SID climbs to a FL but also contains altitude restrictions. Crews would remain on QNH (thus making the altitude restrictions accurately) and then switch to 1013 at the point quoted on the SID chart, ultimately climbing to the FL published on the SID.
Does anyone know of any examples where this is already the case?