I'm flying LAX-BKK in 2 weeks in premium economy. It's $1850 to upgrade to C at the airport. That's far beyond what I'm willing to pay. I think that I'd rather rather use that money to live well well in BKK for 3 weeks than be annoyed with their C seats for 16 hours.

I used to fly TG LAX-BKK often years ago. Back then it was $900 for C to F at the airport (or at an agent.) They advertised $1500, but with minimal effort it could be had for $900.

That was on 7441 configuration. The 7441 F seats were reasonably plush (even in the pitch wasn't good & had no PTV) & service was excellent with caviar & plenty of Dom Perignon.