The change that they provide (the night before) is not as bad as it sounds.
Rather than leaving in the morning (meaning departing your hotel around 8am), you will still have time for a nice dinner in the city, go to the airport, sleep in the plane and arrive early morning with plenty of time for your onward connection. You lose nothing in HKG and save a hotel night. There was no way you could connect in the morning from BKK to HKG and catch your morning AF flight, so you must have been in HKG the night before.
I am not saying that it is what you wished nor optimal, but the differenc of being rescheduled to the night before is not that bad.

If AF had a very low load from BKK, they might have been a bit more open to rerouting you, but they will have so few flights from BKK that they are likely to have big load.
If you really wish to return from BKK, the best option is to get your original ticket refunded and see how much it would cost to have an open jaw, arriving into HKg and returning from BKK.