Originally Posted by
lapo1234
I have a questions about the use of an SDC. I am *G, and if I have a ticket booked for first flight out (6AM) on a Tuesday, can I look to try to move my flight for free to the last flight out (9PM) on a Monday? If so would there be any charge to me if there is availability?
Do you mean *G or UA G? If the former, you're supposed to pay the fee, but the automated systems will evidently let you do it for free. An agent might not be so kind.
The answers to the rest of your questions are in the wiki above.