If you call the day before, the phone agent will only be able to offer a re-confirmed reservation, which entails the change fee and everything. Since this is the first segment of your itinerary, that would result in a reprice, and everything would go south.

You would either just go to the airport early, or United does have a "confirmed standby" offering, described here. I don't use that, so I don't know that much about the details, other than it costs $50 for general members. I don't know if your 1P status gives you a discount or anything - the details may be in the referenced thread.

If there are still seats being sold in the cheap fare buckets, you might just not pay the fee and hang out at the gate for an old-fashioned standby. As a 1P, you should have pretty good priority.