In the old UA I did a same day change to my benefit in the RCC for 2010 qualifying year. I changed from iad-lax on a W fare to iad-las-sfo-lax on a y fare for I knew that the agent was going to rebook me into full Y which gave me the class of service bonus as well.

Now this ISN'T the way it is supposed to work but in the RCC you can get a nice agent that can give you an indirect routing in the old UA system.

Now I have tried this a couple times both on the phone and in person since 3/3 and have been unsuccessful every time. They only allow you to change to the same fare bucket and the same exact routing (although they do allow the 24 hour window whereas before it was 3-4 hours (I think 3) your originally scheduled flight. Thus on UA nowadays I would say this won't work, except in one off cases.