Originally Posted by
Jazzop
In that case, the agent is not supposed to allow you to SDC at all. At least according to some fare rules. A T fare I just bought has a stipulation that to SDC, the new flight must have the same fare class available (i.e., T fare).
I remember it to be this way with NW using the flynow option, but as far as I know with DL there just has to be at least Y1 available to be able SDC onto a flight, even with a purchased T fare.