I don't know how SDC is currently handling broken fares, but that pairing is a Q/U fare for a new ticket, and I suspect that's why you're not getting it as an option, even though the married inventory does indeed include W for both segments. Agents should be able to make the change, as such. However agents seem more and more inexperienced in my current experience, and as a front line they basically use the same website you do, so it may take some trying before you get one that understands what's wrong and what needs to be done to make the change.