Yu Yu Hakusho turned CLAMP
In this doujinshi centered around the main character, Yusuke Urameshi, he and Keiko inadvertently manage to drink some spiked lamune beverage that contained beautification potion from that Utsukushii-whatever-guy. And they somehow turn into CLAMP characters.
A couple more big scans here.
From Yu Yu Hakusho, Yusuke Urameshi Book, by Tatemachikan (1993).