That happened to me last week, I had a FC UG on my regular flight, but the lure of getting home 4 hours earlier was too much to resist. I did the SDC,and within a second, it showed a flight delay that woud get me in later than my orginal flight. I went to the counter and they were able to immediately rebook me on my original flight, had to go on the waitlist for the UG, which later cleared. Even though I had to wait at the airport for a few hours, I am glad that at least got home close to my original flight time that also ended up being delayed.