Ishikei's graphic change can be seen through his TiTiKEi Book, especially the main story.
Same goes for Oh Great when you look his hentai books, even if one had a different style only because it was a job he took under another name to publish to another magazine.
Tomo Chiyoji is another example but a lot less known. His art was very messy looking in the beginning and it was a bit more refined in his later years, despite keeping it's western look.
I'd also mention Bosshi, whose change in style can be seen in the Futabu doujin series.
I'll also briefly mention Saigado, who had a pretty messy art style first then it became smoother.