Thai are advising of a number of changes from A380 to 777-300 on the LHR route:
Thai Airways would like to advise that due to essential aircraft maintenance, our schedule for flights TG910/911 Bangkok-London Bangkok are affected by an aircraft change on certain days, between 15th December 2018 and 30th March 2019.
Rebooking to other dates and refunding (surprisingly) being allowed.