@Martin *******
I previously have booked tickets with Thai Airways which have a departure and return date. I have then cancelled my return flight, and booked another date, sometimes 2 to 3 times. There was a fee of 500 baht each time I did this. Not sure if it is still the same as I haven’t done it for a while. Maybe contact Thai Airways for confirmation. I’m sure it would be cheaper than a flex ticket. Good luck.