Thai Airways' flight change policy +61 1800 005 886allows free flight changes only within a 24-hour grace period, after which you may have to pay a flight change fee to make any changes. When making changes to your flight, you will also have to pay the difference between the old and new fares, taxes and surcharges. Some important things to remember about Thai Airways' flight change policy is that you can make flight changes online only if you bought your ticket directly through the airline's website or mobile app, and tickets bought through a travel agent can only be changed through them.

https://sites.google.com/view/thai-airways-change-flight-ser/home