It's a 13 hour flight so I would probably pay quite a lot to be sitting in J... Depending on how much you paid in the first place, the price differential between Flex and Z is about 3K, between Flex and Lat about 1.5K... (picked randomly)...

So I would probably pay up to $1k and maybe more depending on the situation. This is hypothetical for me anyway as I probably wouldn't go if I had to travel in Y