Great news. Powell is an accomplished composer, and he will add a distinctive style which should separate this standalone movie from the Skywalker episodes. Fans of film music will recognize Powell’s score for the Jason Bourne trilogy, of which The Bourne Supremacy is a one of my favorites.
Personally, I enjoy that Lucasfilm is branching out with the standalone movies, and I’m a huge fan of Giacchino’s score for Rogue One.
Andrew Liptak disagrees:
While John Williams certainly can’t be expected to score every upcoming Star Wars film — he is working on the score for The Last Jedi — it’s incredibly difficult to fill his shoes. Rogue One’s score was a place where the film really fell short, something that a fan edit of the film aptly demonstrated when it replaced Giacchino’s score with that of Williams’
If you read the link article, you’ll notice Liptak misplaces quality for nostalgia. It would be a real shame if Powell attempted to mimic William’s style, instead of writing a John Powell score.