The rules seem to indicate if you have a flight XXX-YYY nonstop that you cannot SDC to a connecting flight.
"You must keep the same origin, destination, thru, and connection cities. Co-terminal changes are not allowed."
In my case all of the nonstops tomorrow are 100% sold out but there is space on later flights with a connection. However I am correct in understanding that you cannot SDC from a nonstop to a connecting flight (even if all the nonstops are full and the connecting flights are offered as an option when booking tickets?)