Yes, real actress on set, face replaced digitally. Probably actual ANH dialog from Carrie herself. I have some minor issues with the CG with these two characters, but I emphasize minor. I have more of an issue with Tarkin’s voice. It was not crisp enough. Too muddled. Both faces have the same problems and it is very common for all cg humans. We notice it less with non-humans. They got the movement of the face close, but not quite and our minds pick up on that. They did a much better job with the animation of Rachel in Blade Runner 2049, but they did a crappy job of making it look like Sean Young. I thought it was a look-a-like actress when I saw the film in the theater. But no, they intended it to be Sean Young and in that respect it was bad. So at least they go the faces right in Rogue One. But have you checked out Superman in the last movie? Caville had a mustache that for some reason he couldn’t shave off and regrow and the CG work they did is horrible. He looks more fake because of his upper lip than Tarkin or Leia do, and he was virtually all real.
I can’t see how you could mistake Rachel in Blade Runner 2049 for a look-alike actress. I thought it was a very close likeness of Sean Young. Closer than Carrie Fisher in Rogue One. Though it still didn’t look real.
But again, both of these problems could have been mitigated by only showing the characters from behind. The audience already knows who they are.