Super Why! (found stop-motion Nick Jr. pilot of PBS Kids CGI animated series; 1999)
From The Lost Media Wiki
Screenshots from the pliot
Super Why! is an animated television show about a kid who jumps into books. The show is created by Blue's Clues creator Angela Santomero in 1999, and a pilot was created for Nick Jr. by Cuppa Coffee Animation in Toronto, Canada.
What makes the pilot different from the final show is that this pilot was animated in stop-motion animation instead of CGI animation, and all the characters looked different from the final pilot. A revised version finally aired on PBS Kids in 2007.