Is it possible to exercise a SDC on a return leg of an itinerary if you have not yet completed the original leg? For example, if the outbound is Day 1 SFO-SEA-SAN, the return is Day 2 SAN-SEA, and while waiting at SEA I would like to exercise a SDC for a different flight on SAN-SEA.

When I contacted AS, the agent seemed to think they cannot modify a RT ticket until the outbound has been completed. So, I would have to wait until I land at SAN, which would leave me fewer choices for SDC's, especially on busy travel days. Their recommendation was to simply book future trips as one ways rather than RT.

I am open to suggestions on how to achieve this.