I frequently do this for PVG/TPE>HKG flights and have never been charged. In fact I almost always book the latest flight and just show up at the airport when it is convenient for me.
I've had the same success as SL, GO, and DM.
I've been turned down a couple times when a flight was cancelled so others had to be moved (no seats available) or when I missed check-in cutoff by a few minutes for a given flight.
Never tried for a long-haul.