They usually book you in a non upgradeable class (not Y,B,M, or H), if you want to change it, you need to go to the airport and have them manually re-ticket each segment in whatever booking class you want.

I did it a few weeks ago. I first went to 1P phone line who told me I had to call travelocity, who then then (finally) told me I had to go to the airport in person to re-ticket.

Best advice is to go straight to the airport and ask them to re-ticket you in a specific class. You will then have to pay the fare difference (in my case it was only about $93 USD.)