I don't really see how this affects flight crew in practise. If it is a sub 980 day your flight will not get cleared to a lower FL than 120 .... in which case you would stay on STD regardless of high/low pressure. Then when you are cleared to 9000' or below you 'transition'. The only difference between this and how LHR do it is that HK bothered to write their procedure on the plate; LHR do it seamlessly in the background .... same system.