My wife and I were in business class NRT-SFO on JAL booked with AA miles last year and on the day of departure got "switched" over to an AA flight leaving around the same time of day. The JAL flight was still going out, no changes at all. We didn't even know until checking in.
AA in Narita got us back onto the JAL flight but it took 4 HOURS to sort it at the airport and we were never told why AA pulled us off the JAL flight and stuck us on theirs.
There was no way we were going to fly in those lousy AA business class seats when JAL has the slanted beds, great service and much better food!
It worked out but was a major headache.
Interestingly, when we boarded the business class upstairs was about 2/3 full, so this was not a case of the flight being oversold.