Do a fake booking on airasia , choose the premium seats, click until u reach the seat select section, check the availability, if 4 seats and above taken then chances are lower. For my case, I saw only one seat is taken, so I actually pay the upgrade one day before my flight for about RM249, ( the later u pay the more u pay). Usually the price is about RM166/way for Seoul. My flight was 2345 and I was being inform of my upgrade around 0000 or 0030? Enough time to pack :)
You will entitle :
20kg check in baggage
1 free meal ,2 bottle of water
Priority boarding vice versa(love this)
Priority baggage unloading
Privacy,long leg room
Good flatbed to sleep
Duvet n pillow provided on board
My Tips: they dont award upgrade base on first come first serve, I'm guessing the more you pay for upgrade , the higher the chances. If you're travelling in a group..chances are lower too. So enjoy your flight!