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. ...
As others have mentioned, passport data can be inputted via the UA smart phone apps. It can handle all passengers on the PNR as a batch.
The manual online entry of passport data has been removed
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?
While not really need, yes you can go to the airport to do this, if you don't use the apps. Probably best to be inside the check-in window, less than 24 hours prior to departure. Then you can use any kiosk or airport agent to do this.