This is certainly different from domestic travel, when standby for an earlier/later flight is allowed on most fares. I believe what UA told you is correct: entirely up to the agents at the airport. I guess they look at loads and decide.

I tried to standby a few times with mixed results. Recently I tried twice at LHR. Once I was told, not problem, it will cost you $200. The other time, I was allowed to go on an earlier direct flight, and transfer my upgrade, no charge. And they did it for two of us!! The flight we were allowed to take was indeed relatively empty.

Another thing to consider with international fares is that you *may* be able to re-ticket your itinerary (with the better connection) with no penalty if the new ticket is more expensive: the trick/hope is to make it go up only a few $$.

Good luck!