It has been playing the same piece of music by composer John Cage at a glacial pace for nearly 25 years. A rare chord change on Wednesday — from seven notes to eight — represented the completion of about 4% of the 639-year endeavor of the John Cage Organ Project Halberstadt. A chord change, done manually, means that the sound of the organ pipes changes either because new sounds are added or existing sounds end. “It’s a very subtle difference,” said Jeffrey Lependorf, executive director of the John Cage Trust in New York. “This John Cage project is a good example of thinking ahead for the next generation — thinking outside the box, beyond the completely normal and traditional,” he said.