londonblue
Topgun Pilot
Also bigger airlines can rebalance the tiers if there's an excess demand in one tier and capacity in another. I've had 2 upgrades on long-haul (dunno how!) because coach was full and there were free seats in business and premium economy (BA).
My brother and a friend once got upgraded from economy to first class on their way home from Las Vegas. I asked him if it was any good. His reply was that he didn't know because he slept the entire way home! (He had been away for the weekend and hadn't actually set foot in his hotel room, so hadn't slept until then.)