I would argue again that some works can belong to multiple genres at the same time: an SF caper film, or a sci-fi detective story or whatever. Somewhere in Time is undeniably a fantasy, a time-travel story, and a swoony, romantic love story, for instance. (It's also been made into a musical.) See also The Ghost and Mrs. Muir. Alien is science fiction (spaceships, aliens) and horror. There's no rule that says any given work is only allowed one label or genre.
And take Alien and the sequel Aliens. The first is very much a horror story with the starship Nostromo replacing the aging gothic mansion, while the second is a military action/adventure story. But they are two pieces of the same overall story-arc in a series of movies. Not only is it a shift in style within the SF genre, it may gave been the smartest move the producers could have made. I think if Aliens had been closer in tone to the original it would have been less remembered - it would have just been just another more-of-the-same sequel.