Good luck on your first l/h C trip!
I haven't flown AY new C yet, only the old one - I don't know which do they operate to BKK. The AY l/h C product is nice, though not world class. Service is "Scandinavian" - relaxed but not over-friendly. Meals are OK, the wine list can have some nice surprises. The (old) seat is showing it's age - not bad, but nowhere near today's "full-flat bed" type seats. The new C seat looks better.
Anyway, if you've flown only Y on l/h, you'll very much appreciate the difference - only problem is, that you'll never want to fly in the back of the bus again

Enjoy!