Thai Airways has various terms and conditions for managing bookings, such as Thai Airways manage booking can be administered for free within 24 hours of booking, if a manage booking request is raised 24 hours after booking, then one has to charge a certain amount as penalty, passengers who book their ticket through coupons or vouchers can be restricted from implementing the manage booking service, if a flight is canceled or delayed due to a certain reason, then airlines leave travelers the choice of canceling their flight. For more detailed information about this visit, Thai Airways official website or check out the blog linked above.
https://www.airwaysmanagebooking.com/thai-airways-manage-booking-policy