Originally Posted by SK AAR

Another option is travel to CNX as booked and buy a return ticket CNX-BKK, i.e. you make a quick turnaround at CNX and book the return segment, BKK-CNX for a date/time at your convenience. Yes, annoying that you have to take a short trip to CNX but at least it will provide the opportunity to stay in BKK for longer (as this is apparently what you are looking for). Domestic flights in Thailand are cheap so it is not going to cost you a lot (but you will of course have to spent spent some time going to the airport, check-in, flying to CNX and reverting immediately to BKK etc)