You should be looking at the booking info on "flight status" not fare class availability.
839 is booked full 4/3. It's booked 34/40 for 4/4. 863 is booked 37/40 and 38/40.
So your current flight has the *best* upgrade chances. Though that is a relative term, as the odds are, as previous posters have noted, not very good at all.
As WineCountryUA notes, UA will not fill F in order to allow upgrades Y to J.