I was able to do this in reverse recently: I changed the outbound without affecting the return flight (and my cleared upgrade). I was reluctant to try it on-line for fear of the system messing with the return flight, but an agent handled it without incident.

You can use the online tool ("change my flight") and see if there will be any fees involved - just go as far as listing the available flights, and no further. Then call the agent with the info on the flight you want.