When on our DONE4 trip last year, we were at NRT, awaiting a CX flight to HKG, and when we checked in, we were asked, since we were in time, would we like to get the earlier flight!

We had just collected our tickets from the AA ticket desk at the airport a few minutes before, so were at the airport very early for our afternoon flight.

I also changed at LHR from CX to BA flying LHR-HKG (for a flight the next day). Easy to do, and less than 24 hours notice, using the AA ticket desk at the airport.