As I understand it, one cannot check in online and receive their boarding pass if using a new passport. The new passport needs to be swiped at check in at the airport.
If this is correct, can one go to the airport prior to the day departure and have this new passport added to the profile/PNR?
If you have the United APP on your phone use that to check it. When it asks for your passport data and has the old info - do the route for adding another passport. You then use the camera to photo the data page (just like a scanner) and it will electronically convert the data. Confirm the data and proceed. I have two passports and do this regularly.