Friend of mine recently flew SFO-HND on UA875/876 which was on the 787-9 with the old 2-2-2 config in Polaris, but I noticed that the same route is on a 787-10 with 1-2-1 in Polaris with PremiumPlus for sale in early Jan. ...
Aircraft type changes occur for many reasons including seasonal rotations, capacity shifts, one-off changes due to irrops, ....
Is there a place to view when aircraft are "officially" changed for a route?
UA sometimes announces but more commonly does it passively.
There are sites like
https://www.routesonline.com that detect and reports
such formal changes
UA changes