I'm being picky here, but "ballad" isn't a genre. And you can get ballads in every genre (e.g. rock ballad, pop ballad, RnB ballad, hip hop ballad, dance ballad, etc.). So you can change an up-tempo song to a ballad, and have it still in the same genre.
Also, it depends how picky you are on determining what a "cover version" is. Since you could include literally millions of hip hop songs that heavily sample songs from othe genres.