American Airlines upgrades from coach are to business (or first) class. They skip over premium economy, which is a good thing. However that’s also meant there isn’t a way to upgrade to premium economy in advance.
The airline has a day of departure upgrade program for premium economy, and Executive Platinum and Concierge Key members have received these seats free when they are available (and when they aren’t being upgraded to business class). This was later extended to Platinum Pro members as well.
For Alaska and Hawaii routes, these complimentary premium economy upgrades at the gate are now available to all elites. Gold and Platinum members are now eligible for complimentary premium economy upgrades when listed for an upgrade to business and not clearing.
Agents are supposed to ask before upgrading a customer to a premium economy middle seat.
In practice, premium economy upgrades are one of the things most processed incorrectly. They aren’t uniformly understood, and there’s little time at the gate when agents are under D0 pressure to get a flight out to call up a customer and ask them whether they want an upgrade before moving onto the next customer.