I rarely standby for a later same day flight (which is why I reserve the latest flight in the day that I might want to travel on) but when I have, I have always cancelled my existing reservation and let them know I will be standing by (always check ITN to make sure there are lots of seats before doing this).

Can't remember if I have done this at the airport or reservations or both. Reservations will cancel but I don't think they can put you on a standby list.

As a SE or E it is easier but if I was a basic member I would be retiscent to standby for a later flight.