The outcome: they upgraded the earlier flight from a 319 to 320, which happens to share the flight number with the NRT flight. This added 4 seats in F. When I checked around 12 hours before the flight I found it to be F4 but also showing 4 for NC and XC inventory, but XF 0 and NF 2. (I thought this was as an interesting observation).

So I called the 1k desk and they cleared me straight into F.

It turns out that I would have been ok on UA 339 as it arrived 10 minutes early. Prior to boarding, I chatted with the gate agent and a fellow who looked like a supervisor. As expected, they encouraged me to take the earlier flight, especially when checking a bag.

I had monitored the three previous days. One day it was on time (maybe a few minutes early), one day 30 min late (so would have missed flight) and one day 15 min late (would have just made it but prob not bag). The December performance is 85% on time, so that's pretty good. Next time, I think I'll give 339 a shot. DEN security is also quite miserable around 7:30 AM with long lines even for elite.

One negative about taking UA 875 all the way through is earning fewer miles.