AAgent is correct. Non-elites can use Same-Day Flight Change for $75 or Same-Day Standby for $75 (refunded if the earlier flight is full) to go on an earlier flight.

Agents may be willing to rebook onto earlier flights in the event of lengthy delays, generally over 4 hours, which is typically when meal vouchers start being issued. It's not an official policy for the reasons AtlanticX pointed out.