Wow, there are some confusing answers here. Let me add my attempt.

As a GLD, you can do a confirmed same day change to the later flight, but only if E inventory is available. If it is, it will cost $75 and you'll be confirmed. This option is available within 24 hours of the desired new flight time.

As a GLD, you can also do free standby for the later flight. Call AA before departure of the original flight to let them know your intentions. When you do this, you can ask the phone agent how the loads look and how likely you are to clear. Once you have called, there is no need to be at the airport for the original flight, just arrive at the airport for the later flight, maybe allow a little extra time to get your pass to get thru security. Except in rare cases, you won't clear until boarding time, and maybe not until right before doors close. If you don't clear, you'll roll to the standby list for the next flight, and so on.

https://www.aa.com/i18n/utility/sameDayTravel.jsp