I had one good experience through CX in BKK. I was traveling on a LONE4 and asked the agent at CX's downtown office in BKK to change a KUL-HKG sector I would fly a couple of days later. I was originally booked on the afternoon service and wanted to travel on the evening one (same day). L was zeroed out, but the agent tried to request a L-seat anyway and it cleared immediately.

I was unsuccessful later on the trip though. I tried to move my NAS-LHR sector on BA in order to fly one week earlier. Both BA and AA in ORD were unable to do this and the waitlist on the earlier flight never cleared although I enquired more than three weeks before the flight (of course NAS-LHR is much more difficult to clear than KUL-HKG).

So I was lucky with CX, but unlucky with AA and BA.