The only "free" upgrade you can get on intl is an op-up: see
here for more info on those. In brief: if economy is full and J is quite open, UA might oversell the back and then op-up elites to the front. Does happen on US<>Asia routes around peak leisure travel season, so your odds wouldn't be zero ex-TPE at Christmas, but aside from a few "op-ups often happen" routes (ex. the old SIN<>HKG), you should consider it a unicorn-type event.