I have never heard of any airline allowing voluntary standby on any international flight, at least not in Area 1 and Area 2.

However, what other posters are encountering is the occasional ability to reconfirm to an earlier flight without paying any penalty. Because of security and baggage, allowing passengers to standby for international flights would be far too complicated to allow for an on-time operation. However, if you get lucky and an agent is willing to *confirm* you on an earlier flight, then you're golden.