Retired, but was a captain - both long and shorthaul.
Unless it's changed, which I doubt, no passenger is ever downgraded to allow positioning crew to sit in a higher class than a downgraded passenger.
IT DOES NOT HAPPEN.
I think Henky started this (if it wasn't you, accept my apologies).
What I suspect happened was he overheard someone who had asked for an upgrade whingeing because he hadn't got it.
In my experience they fall into several categories
Don't you know I'm an MP, euro MP, government minister etc. - they have bought an economy ticket but have charged the tax payer for a J or F ticket.
They are a personal friend of Lord King (you can't be, he's dead), Colin Marshall, .... Willie Walsh (you can't be - he doesn't have any friends, or at least that's what some seem to believe).
And my all time best, I am a personal friend of the Archbishop of Canterbury (Robert Runcie at the time).
To which the first class steward replied: "you had better pray for divine intervention then".
I will write it large again IT DOES NOT HAPPEN.