Since the FTH for +12 was 10,000 feet or so, it would be pretty reckless not to ground test at +12 to ensure that Mod. No. Merlin/154 was embodied correctly given that it was a local mod; otherwise there would be no guarantee that +12 might not be exceeded, leading to rapid failure in flight.
Reverting to 87 octane would require that the 87 octane limits be observed; since operation of the cutout would now give +12 it would probably be wired shut; clearly in this condition the aeroplane would not be exactly combat ready.
Having filled the tanks with 87 octane again, they would probably require cleaning when reverting to 100 octane to ensure that the fuel reaching the engine met the performance standard.
Changing fuel is not quite so small an undertaking as many would imagine; aeroplanes are less forgiving than cars, and the consequences of engine failure are inevitably more severe.
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