Let's say you're booked on a 7PM flight and do a confirmed same day change to the 1PM flight. If you miss the 1PM flight for whatever reason, are you back to flying standby the rest of the day?
I'm guessing "yes" since you've given up your seat on the 7PM. If the 7PM was oversold, you wouldn't even be able to reconfirm your seat back with CSDC. Right?