This isn't going to change the core background grim darkness, or the vast majority of stuff Black Library puts out. If you read the short stories from the Malign Portents website, you can see that Age of Sigmar is going in a darker direction. They even changed the logo to be darker.
In the new edition the core book is going to have a much more filled in explanation of how things went from the End Times through the Age of Myth and the Age of Chaos, to how we got to the Age of Sigmar, and how the realms are laid out. Stuff that really should have been in the first edition, as it was so vague in the first edition, and so much misinformation became what people thought was real