As a general point, the American concept of "being on standby for an earlier flight" is something that doesn't really exist in Europe. When you book a flight you generally have to expect to travel as booked.

As others have said, if you're on a flexible ticket then there is nothing stopping you changing your reservation to an earlier flight subject to availability, and as long as you have no checked baggage you can change your reservation at any time up to the time of departure. However, on a non-flexible ticket there is no way you can stand by for an earlier flight.