That's what I thought when I started, however, when I got in there, what I found is that that doesn't seem to be a thing in the K1 portraits. It's basically just lighting and skin changes. The transition 'facial expression' changes, such as they are, are actually from the Dark-side version of the textures having different contouring and eyebrows painted on (especially in the female portraits). It's incredibly awkward in some cases, because the vanilla skins between extreme Light and extreme Dark were just layered on top of each other and blended at 25%, 50%, 75%. It caused some bad results for some of the faces, including my favorite one, PFHB02, where they got the eye position wrong in the extreme DS version of the texture, so during the transition the iris kind of drifts off, and in the 50% blended version, the person has two iris/corneas next to each other. That was something I had to fix. PFHC01 had the same deal.
That said, early on, a few portraits looked like they had different facial expressions that weren't painted on, and for those I did 3 different pose levels across the 5 portraits. At this point, I don't believe they actually did have any expression changes, but at the time I did. I mean, look at PMHB2--they left him smiling (not even creepily) in his extreme DS portrait, if they were going to change any expressions, that would have been a good candidate... I felt pretty weird leaving it that way, but... vanilla is what it is.
I did lighting changes on many but not all of the transitions, probably not as many as would have warranted them. 150 different renders already took a pretty huge amount of work so I did have to phone it in on some level.
The TSL PC portraits seem to have clear and obvious facial expression changes with little/no lighting changes. I haven't started that project though so I can't say for sure if that applies to all of them...
I could, and the body textures would improve the clothing a bit, however, in the couple tests I just did, the faces don't seem to be positively changed by using the upscaled textures. The AI doesn't seem particularly good at skin and hair. They don't get a lot worse either though ... it seems to be, do you want your eyebrows to look pixelated, or do you want them to look like watercolor paint? Definitely not an improvement on a level that would get me to rerender everything.