Specify different fare class for each leg
 

So it does not appear I can do it online without buying two one way tickets. If I were to book over the phone can I specify the lowest econ fare one way and M fare (for m-up) on the return with a res agent? I have an upcoming trip from IAH-ANC-IAH and would like to get 1st on the way back.

Also when you do search online and you see the yellow U for Y/B/M up. Does that mean the seat IS available for upgrade or simply that you are purchasing a fare that will make you eligable for the upgrade but does not mean there is inventory for it?

Put a reservation on hold using the cash option and then call in and have the agent change the fare class of one of the legs. I have done this a number of times.

Means it's available if you qualify. You'll see Ms and Bs that don't have that yellow tag...that means that the appropriate inventory is zeroed out.