It may not be a sequel. As Nagumo mentioned, the game is called White Night Concerto in Japan. It was only when it was brought over to the states that it became Harmony of Dissonance. So either,
a. it's a whole new game, and they choce Harmony because it hasn't been used yet in Japan
b. the title trademarked is the US one, for whatever reason
I would still like to believe it's a whole separate game, and that what we're seeing is only the Japanese title and will change upon release in the states.