Same Day Flight Change at Airport (Standby to earlier flight)

I tried searching, but was getting a ton of non-related input from the boards, so I wanted to see what experiences everyone else has had.

On Monday 2/25, I will be flying from PHX to EWR on the 5:40 PM flight. My company put me on that flight, however it gets in just after Midnight. The two other options are 12:06 PM or 7:10 AM, which I would prefer to be on.

Has anyone had luck with just arriving at the airport REALLY early and trying to get onto an earlier flight for free? I am a Silver, so I would have to pay $75 otherwise, but was wondering if gate agents are more lenient than phone agents.