This is not true. There is nothing rigid about music. You can do whatever you want. Unless you care about being popular. Then what you run up against is what people enjoy.
You can certainly do what ever you want. But there are distinct rigid structures to all forms of music. If I make a close approximation to B.B. King's "3'o'clock Blues". It is most certainly Blues and not a Polka song or Reggae. Even Avant-garde Jazz has a common theme. Music is formulaic.
