I've also managed this once on a BA Avios redemption booking when at the lounge in Incheon. I suggested the change so as to avoid a 4 hour wait in the lounge and to get onto a plane to HK with long-haul configured seats. The lounge agent were more than happy to oblige after a brief discussion with the station manager. When I checked the loads, I could see that my original flight was fully booked in J, whilst the earlier flight was wide open. So, YMMV.