The piece consists of a melody by the French Romantic composer Charles Gounod that he superimposed over an only very slightly changed version of the Prelude No. 1 in C major, BWV 846, from Book I of J.S. Bach's The Well-Tempered Clavier, published in 1722. (emphasis mine)
What are the slight changes between Gounod's Ave Maria and Bach's Prelude No. 1 in C major?