Just a quick update. I couldn’t find answers for this easily when I was searching myself so I thought I would share.
In the event that you book an international flight on United and then get a new passport (you ran out of pages, renewed it, etc.) you can’t update that number online during check-in. However, after a call with United’s phone support here’s what I found.
Go ahead and check-in online as you normally would, use the old passport number if it’s already entered. This is where you can’t change it. Then, when you go to the airport, use the kiosk, bring your old (now hole-punched) passport as a backup, and check in with the kiosk. You should be able to scan your new passport and it will update your United records for all flights moving forward.
Hope it helps!
The United phone line is 1-800-864-8331. You’ll ask for existing reservations and help with something else to see an agent.