As far as I know, you're out of luck. You would have options if it were US metal. But with a US flight # and UA metal, I am not aware of an upgrade mechanism for you, besides buying up to first class.

"A" class fares (discounted FC fares like QUAUP fares) are frequently rather reasonable for domestic travel with sufficient lead times and are frequently offered for US flight #/UA metal and UA flight #/US metal flights. If upgrading one direction is more important than the other, you can also upfare one-way and sympathetic agents will often void the change penalty.