However, our SOPs (and most others I believe) say: Select new level or alt on MCP, then change subscale, then initiate climb or descent.
I would agree with that. On a practical note, it is useful to brief for unusually high/low QNH and to work out what effect it will have before you get to press the button! If you keep the rate of climb/descent down while close to transition, you might avoid an inadvertent "bust" as well. In the last few weeks we have had QNHs from 970-ish to 1045+ in the London FIR...