References to “old artwork” in any official materials might be referring to older 3D renders. The player character’s 3D model was updated mid-development, so portraits and CGs made earlier look different than those made later. At one point I planned to update this older artwork with the new 3D model but I no longer plan to do that.
The 2D art was never planned to be updated in that way, though the later 2D art does look somewhat different than earlier art as well for certain reasons (one being that the artist kept “buffing” her, if you know what I mean). There was more time to work on later art in general, leading to later 2D and 3D art both being of higher quality than earlier efforts.