Disney was already an all-star animation team when Toy Story was released. But this was the first-ever feature film to be fully computer animated, so it was a whole new ballgame. It was like inventing a new ballgame, while they were already on the field.
The end result was masterful, but there were growing pains along the way. Looking at early concept art, you can see where they designed characters to fit the old Disney mold, where they shot off in wild new directions, and where they honed these new ideas into the familiar designs we recognize today:
Source: Character Design References