Had an interesting experience:
Was going ORD-SFO on an L. A SDC ORD-SAN-SFO would have worked out better in my schedule, it showed up as an SDC option, and L's were available on both segments. However, the change wouldn't go through. I called, and they couldn't make it go through either. The reason they told me was although there was L inventory on both segments, there wasn't any L inventory on the combination.