Standing by for a completely different flight altogether isn't like standing by for an upgrade, where the system automatically checks for availability and automatically confirms you in the premium cabin.

Standby is cleared manually by the gate agent, with the exception of extremely lightly loaded flights in which case it's possible to receive a seat as soon as you're listed. Theoretically the latest time at which you can clear is at D-10 when the doors close.

You can cancel the standby request if you don't like your odds, but if your original flight has already departed it doesn't leave you with a lot of other options.