A family crisis means that I have to bring my trip to Australia forward to ASAP and I'm wondering what my chances are of standing-by on either of the BA flights to Sydney?

My ticket allows date changes but is slightly restrictive and I'm wondering if I should just buy a new ticket and use this one later, or if I stand a good chance of getting on a plane as a stand-by.

I've never been stand-by before ... what does one do? Go up the check-in agent and tell them you want to stand-by? Or to the ticket desk?

Thanks