This is one of the things that CONTINUALLY burns my bacon about united.com! Two examples:
I would fly MKE-ORD-SFO, with the MKE-ORD on an exPlus CR7. I would select "Upgrade All Legs MKE-SFO" on .bomb, and it would say "You need 5 e500s to upgrade this itinerary."
No, I don't!
Example II: I got a cool routing of MKE-ORD-SEA-SFO one time, and again, tried to upgrade. Long story short, I spent 15+ minutes on hold while someone went to get a supervisor. The phone agent did not believe me when I told him that e500 upgrade mileage goes from start to finish, not a-b-c-d.
To his credit, he apologized, said I was right after all, and thanked me.