I’ve just checked my flights for my trip in October. The return from Seattle was specially chosen to include the UA 747-400 from SFO-LHR. This has changed in to a 777. This is disappointing but not all that uncommon for the winter.
So, if you have a flight booked for the autumn/spring, I would check you seats. The flight time has gone up about 15 minutes, cutting my connection time. Upgrade intact!