They tried to unify the looks of characters across the trilogy by using ME3 character models in all three games for some characters. Even Miranda looks slightly different in ME2.
A BioWare dev once said that they had problems importing the Kelly character model in ME3, so they tried to recreate her from scratch with the available hairstyles of ME3. They then took the character model from ME3 for the remaster, but also changed a whole lot of hairstyles, so many characters ended up looking quite different.
One noticeable hairstyle change can be seen in Jacob's loyalty mission, where some of the women had greasy looking hair. That hairstyle is gone, and now they look like they haven't spent 10 years on a deserted planet. Some other characters, like Nef and Dr. Kenson also look different.
Funnily enough, despite people pointing out at ME3's launch that Kirrahe looked different from his ME1 appearance, they didn't fix that. Kirrahe has different face markings in each game.