In the Warrior Cats graphic novels, illustrator James Barry's art has gotten noticeably better between his first trilogy and his most recent volumes. While in some of the older works, the cats more or less had Only Six Faces, but now, in addition to being better art in general, the cats are a lot easier to tell apart and they all have distinctive facial features.
An interesting in-setting example is shown in the novel My Name Is Red, in which the traditional Ottoman/Persian style of art is being influenced by European and Chinese styles. The reaction to these changes are a major impetus for the novel's plot.
Mary Grandpre's illustrations for the Harry Potter series became less geometric and way more realistic over time.
The Rainbow Magic series' art has gotten cleaner over time, and much more detailed.
In Little Critter, the title character was originally stockier and more rodent-like. He later became thinner and cuter with a bulbous nose.