Of course, if economy is overbooked then a staff/ fellow crew upgrade frees a seat in peasant class thus allowing a standby fare payer to travel.
Contentment all around? Above all, keep it low keyed and polite and try not to let fellow passengers twig.
I usually identify myself as crew at the gate or to the Chief Purser immediatley upon boarding. Once down the aisle as it were on a full flight; I usually find that the poor overworked cabin crew have enough on their plates.
