The same happen to me a few weeks ago ex BKK. I checked in online to discover I had been moved away from my family from my prized 62 A down stairs to 17 A!! Horrified I called Gold Card but to no avail. No explaination. I was furious at the time. Following a restless night I checked again and I and my family were op up into First.
The flight was oversold from Sydney.
As an aside this had happened on the two previuso occasions; online upgrade and the first time at the lounge...
I guess an op up is on its way. Good luck!!