I've done standby for a later flight a few times. I nearly failed to clear on the last flight of the day once. The agent said he suspected I would be rolled over to the first flight in the morning with the other standbys that don't clear (never mentioned being rebooked; I wouldn't expect that since your maneuver is all voluntary.) I can't confirm this would've actually happened though. If you can SDC online to the later flight you want but don't want to pay the fee, standing by comes with considerably less risk (as long as there are no ops issues for earlier flights.)
flyingeph12's advice matches my experience - It's very important to call ahead to let AA know you intend to standby for a later flight. The agent does some work to cancel your ticket but makes it easier for the check-in agent to pull it back up and put you on standby when you arrive at the airport. Sometimes phone agents aren't familiar with the later flight standby rules and say this can only be done at the airport. You may want to consider calling again in these cases so that nothing goes wrong, or you can also request your standby flight online. Also make sure to check-in exactly 24 hours before your original flight so you can be at the top of the voluntary standby list at your respective status level for the later flight.