Maybe start by asking if they would look up the BKK-CEI segment first. You should be able to tell if that goes smoothly and if you have a good agent.

I had 3 similar award reservations and needed to make a change to one of them. The representative did not listen and simply took the first and made the change. It wasn't until I had hung up that I saw the mistake. Fortunately, the original flight had plenty of availability and the next agent put me back without issue (and refunded the change fee).

Looking back I should have known the first agent would not get it right but it is hard to say why. It's maybe that some of the best agents are very meticulous and accommodating versus a cavalier attitude of "yes, we'll fix it, say no more".