I actually witnessed a very similar scenario, just across the aisle from me, in the F cabin of one of my EK flights a couple of months ago.

By all accounts, two pax out of a group of three had been upped from an overbooked J cabin (the 'chain' had presumably begun in Y as so often can happen...). One of the two was keen to offer the unexpected benefit to his travel companion/relative (ie third member of said group), seemingly because he himself was not especially excited to sit in F but felt that the other person would be.

He sensibly first asked an FA if this was permitted. FA said she would need to refer to Purser. Purser appears, and says very politely - but very firmly - yes, you may switch seats BUT you can only do so once - and now please, before take-off (aircraft was still on the ground) and cannot do so again during the flight.