You are unlikely to get the F seat on the UA segment as UA would have confirmed all its own elites into those seats before it would even consider moving you up on the basis of your ticket code. UA now upgrades automatically all its elites on NAmerican flights based on status. If there are 12 UA elites on that flight, they'll all get F seats ahead of any consideration by the gate agent to move you up. While rules of the ticket say you can't move up on a STAR partner, UA agents have lots more leeway in these matters, so wouldn't hurt to ask when you get to the gate. Check the overhead board to see the list of upgraders pending.