I don't think the original poster is asking about the definition of a direct flight. It seems that he is asking if one can go standby on a direct (or ns) flight as opposed to the ticketed flight.

The specific issue here is as follows:

UA often has special fares to and from ORD. So a ticket LAX-ORD-BOS-ORD-LAX may actually be calculated as two separate tickets (LAX-ORD-LAX and ORD-BOS-ORD). The total of these two is often less than LAX-BOS-LAX.

Follow?

So changing the routing would mess up the fare basis.

I have never been able to change a ticket in this situation in advance without add/ collect. But I have been able to do it at departure. It all depends on the agent and his/ her authority to do this.