So I never got around to buy Sun/Moon, was about to when they announced USUM. So I have a question for you.

Is USUM a slightly modified story, like gold/silver and crystal, or a whole new story like black/white and black/white 2?

That's all I care about. I don't care about weird evolutions or multiplayer mumbo jumbo. I just care about the story. Will I miss something important about it if I skip Sun/Moon?