Thanks guys. On UA 2, 30 mins from arrival. The app gives me 4 options, 2 of which are for flights that have already left, the other 2 gives me connections in ORD that will get me into EWR after my originally booked flight. So all 4 options useless.

Before I boarded my flight, I did see on expert flyer that there were 2 other direct flights that had economy seats left that will get me into EWR early.

Sounds like from comments above, I should try the kiosks to see if I can add myself to standby? Failing which, I'll try gate agents?