Standby (free for elites) and confirmed flight change ($75, available to anyone) are very different things.
As an AA Gold you can standby for an earlier flight without a problem, but you must request that at the airport (club is fine). You will clear based on your position on the standby list and the number of open seats. There is nothing you can do now except read up on the differences and the standby process, I suppose. Or just browse FT idly
Hope you get home sooner!