I just went through this earlier this week flying back from overseas. The moment I landed, I called up the Diamond line to SDC to a different connecting domestic flight. Both legs (int'l+domestic) had cleared weeks ago with an RUC. The phone rep pushed back saying the RUC is just like a complimentary upgrade and, thus, the same fare class as originally purchased must be available to SDC. But I held firm and asked for a supervisor and the rep said she would check and get back to me. When she came back on the line, she said I had been put into first on the flight I wanted and was good to go.