As far as we know, AY normally schedules the older A340s on the BKK route every day. An AC change for operational reasons is still possible, but perhaps not that likely. They made some kind of "promise", ie they have marketed, to use the new J on certain routes.
Cargo loadfactor + distance dictates A340 on some routes, so if they suddenly have very high LF they might need the last 340s even on the routes where they promised not to use them. OTOH, as they no longer promise new J on HKG route (and never did on SIN), I can't really see it would happen.