I guess as a last-ditch effort I could just cancel it, get a credit and then re-book into premium economy but that seems kind of....dumb. Am I missing something here?
Just that AA IT is notoriously bad, glitchy, wonky, weird, etc. If PE isn't showing up trying to change flights online then you can either just call AA and have an agent make the upfare or cancel/rebook with a credit as you mentioned.