...In Music History? I would have to posit that the Romantic period of music changed the way we view the world today. This was the period that artists stood up to oppressive views of the Aristocratic and Realism. In a sense, they were the rockers of their day. I say that this period changed so many things because it lead to the industrial revolution, emotionally based artwork, and freedom of expression.
Dont get me wrong. I do believe that every time Hendrix or Stevie Ray Vaughn lifted the guitar history was rewritten, but when you're asking about music history you have to be more arabiguous.