Not my experience at all for the most part - I have been moved onto earlier flights several times, usually ex-USA where they are supposedly even more officious. I'd say I have asked about 10 times, of which 4 the previous flight was full in my cabin, 1 they said outright no, and the rest they either did it at check-in or it came through in the lounge once they confirmed the loads on the earlier flight. Might help being GCH, usually flying CW and hand luggage, high CIV.

I have found several US carriers which supposedly do this more freely to be much tougher on this, and have ended up reaping lots of comp vouchers, upgrades and miles after being massively delayed because of their obstinacy - DL, UA and US. On the other hand, I find AA routinely let me stand by for earlier domestic flights as an OWE without any fuss at all.