UA's policy is that they don't let customers double-upgrade to first class. Instead, they fill first class with their own employees and give upgrading customers slightly worse service in business class than they're used to geting when upgrading in two-class aircraft. Oh, yes. In UA's three-class domestic flights it's not that they add a class better than the two-class flights. They add a class in between the two and put you there even though you're paying the same to upgrade. It's frustrating and one of the two worst things about UA in my opinion, the other being a refusal to let elites upgrade any fare internationally.