I checked again today, and a seat had opened up, so I should be good (and won't be denying someone else boarding / downgrading them due to my ticket ).

The overall ticket is to SNA, via SFO, and I'm still amazed that a full F fare there is as cheap as it is (the IAD-SFO segments are ~$600 PQD, and the SFO-SNA ones ~$100). ETA: I see it's still an /UPDI fare, but which books into F rather than A, P, Z, etc. Interesting.