Screenshots
DT is on the left, 6.58 on the right. I tested most race+gender combos except lala and hroth. Most race+gender combos except M Roe had at least one face whose lip color shape seemed to change substantially.
I mostly tried to use pale skin and dark black lip color.
Viera
For both male and female Viera, the lower lip appears to be rounded out. For F this results in a plumper lower lip and a wider apparent angle at the corner of the mouth. For M this makes their mouths appear substantially more effeminate.
Miqote
The rounded lower lip appears on Miqote as well, though it's more noticeable on the M. For F she used to have something of a smirk or :3 face; now she just looks pissed/serious.
Elezen
F has the rounded out lower lip that I'm seeing a lot, in addition to a very thinned upper lip. M's seems to cover the entire lip up to the corners rather than stop short.
Highlander
I quite like the changes to F, though again there's a rounded out lower lip. M's has been smoothed out and the lower lip fattened.
Au Ra
M was the only one where I had to use dark skin and light lip color; pale+black was almost invisible... Anyway, again, rounded bottom, and the top outlines the cupid's bow a lot more. For F, her bottom lip appears much fatter and she looks like she's "puckering up" -- I think what happened was in 6.58 the lip color application didn't reach all the way to the "fold" of the lower lip, while in DT it does.
Midlander
Both once again have a rounded lower lip, and both also have thinner upper lip color.
Femroe
THIS is why I made this thread! Just look at my girl!
It's actually more obvious at an oblique angle but she looks older now (or "more mature" you could say). The lower lip is significantly rounded off. The upper lip seems to have a sharper cupid's bow -- some of the color seems to extend beyond the fold at the points. I wouldn't mind this as an option but I'd like the old shape back.