Slight update to this,
UA only just uploaded the 2nd SFO, but is now downgauged to a 772 and is operating on a shorter season from early December, starting off at 3x weekly (ramping to daily from Dec 17) until March when it starts winding down frequencies.
The year round UA863/870 is also getting downgauged to a 772 for NW25/26 per the UA booking engine directly.
The 2nd SFO previously operated for the entire NW season but has been cut back for the upcoming NW season with a smaller aircraft (assumedly in light of the lower than expected loads and are 'right sizing').