The rules require an SDC to keep the same routing, but in practice it's agent dependent. For example, I called once to try to change a BOS-SEA-SFO to BOS-SFO and got denied by the first agent, but called again that night and was able to make the change. I believe if you can change the routing you accrue more miles for it, and I think some coterminals are allowed (for example SFO/SJC/OAK) but am not sure.