LH Group carriers generally offer much more approachable paid upgrades to J than UA, including both flat fee upgrades and the "bid for upgrade" option. It's just a different revenue strategy. As I said, you may or may not be able to purchase an upgrade from UA. I'll add that it may or may not be at a price that seems reasonable. Often it is not. Personally, I'd never buy a long-haul UA operated economy ticked with any expectation I'd be able to buy a cheap upgrade. It's just not predictable and many times will never be offered.