Not quite answering the OP's question, but I've done same-day changes with PG on routes where they have hourly flights to/from BKK and they've been accommodating.

No change fees, but in some cases where I've turned a connection into a longer "stopover", they have printed me forms where I was now subject to additional airport fees or taxes. Not sure exactly what that entails, but nobody has collected from me yet.

Coincidently, I​​​​ have a 5 pm flight today, and will try to get on an earlier flight.