You need to continue to watch your seats when originating from a HUB airport right up to and including 2-3 hours before departure due to last minute aircraft swaps. On 02/22 SFO-NRT, I was flying an inbound flight to SFO and about 3 hours before my actual SFO-NRT departure, they switched the aircraft from a new PE aircraft to back a non-PE aircraft. Luckily I was refreshing the UA mobile app every half hour or so during the flight, or we would have been knocked back to some non-desirable seats. I was able to switch mid-flight.