However, I don't think the phone agent can actually put you on standby. The only way the phone agent can change your flight is if you pay the $100 change fee and are confirmed on the earlier flight.

Otherwise, they can tell you that the earlier flights look OK for standby or not. Just remember that anyone paying $100 to change their ticket to that earlier flight will receive a seat (assuming it's available) and the numbers of seats available will go down.

-RM