The last few posts are correct.

UA, sometime "recently", started OpUp'ing people by seat assignment only and not changing the fare class on the reservation. This means the app will show an under booked Polaris/First/Business cabin even though all seats are occupied. This makes the "availability" section of the United app useless in these situations. It also means you can't go into the app and then SDC using your newfound "J" ticket to move to other flights more advantageous to your liking.

As far as I can tell this new policy is for all flights, domestic and international. I've seen it firsthand on both types.

-RM