For a recent flight, booked on a three class 772, I had seat 10H. Two days prior to the flight, plane was switched to a Polaris converted 772. United assigned me seat 11G. I switched my seat to 3A. My wife was upgraded at that point and chose 3L. Then in the morning of our flight, the plane was switched back to the three class 772. United assigned me seat 10A and my wife had no seat. At the airport, after a lot of permutations (changes made at the check-in desk, Polaris Lounge and at the gate), I got 10C and my wife 6J.

On the return flight, booked on a three class 772, I had seat 7H and my wife seat 6H. Two days prior to the flight, plane was switched to a Polaris converted 772. United assigned us seats 4D and 3G. We immediately switched our seats to 3A and 3L.

For both flights, when the switch was made from a three class 772 to the Polaris converted plane, the seats in the middle were all assigned, leaving many empty window seats that then could easily be chosen.