Yes you can do it. I actually did this last week and they even call it out on the website. I originally called customer service 8 hours before the flight and they said I had to do it at the airport. When I talked to customer service at the airport, they said the computer said I couldn't, but when I went to gate, the agent cleared me onto the flight even before boarding started.

Can I change my routing on the same day of travel? Yes, you can do so when a change can be confirmed, but only if the fare allows for the new routing and there are no changes to the origin or destination airports. You can’t change your routing when you’re standing by.

I only found these FAQs after the first 2 rejections from the customer service reps, and was going to bring it up if I got one more rejection. https://www.united.com/ual/en/us/fly/reservations/flight-changes.html