When Walt Disney produced the original Fantasia, he had intended for it to be something that he could change over time. New sequences could be added in and removed at Walt's will. Unfortunately, the film was not as commercially successful as he had hoped and all such plans were canceled.
When the 1990's video release was a huge moneymaker, Walt Disney's nephew decided to take some of the profits and make Walt's dream come true. The result was Fantasia 2000.