depends. Is your itinerary booked as a 1 way or as a round trip. If you throw away the HKG-BKK sector and it is in the middle of a round trip then you will lose your return (most likely).
If it was me I would just call AA and change the itinerary (and pay the fee). Mileage requirement is the same since REP or BKK is the same region. Save the Avios for another trip.