TG676/677 uses aircraft that turn around from CDG-BKK flight. If that aircraft is delayed or gone tech they will use aircraft that come back from CPH instead. Again, this is a trade off between lengthy delay vs seat comfort. I'm sure that if the delay is short then TG would not change aircraft but if the flight is delayed, say, 3 hours then swapping to 747 and leave on time or short delay will make more people happy.
Moreover HS-TGN had gone tech at LHR a few days ago and remained AOG overnight. That must wreck havoc on TG's tight 744 aircraft rotation and might have played some role on your unfortunate aircraft swap.
Very useful, insightful info-thanks for posting ^