Alexandre Desplat confirmed that he is indeed composing Rogue One:
Alexandre Desplat. That is who everyone was told is composing the music for Rogue One: A Star Wars Story. That very well may be true and I honestly hope it is. If I had to bet, I would think in the end it would be Desplat if it already isn’t. Very recently while talking to some friends in the business we heard some interesting things about Rogue One and “the composer situation.” While investigating where the recordings for Rogue One’s score would take place we heard this rumor from a trustworthy source close to the activity:
There’s some business issues. At this point there is no contract in place and it is a bit contentious. They’re looking at two other composers for the film should it go another way.
Alexandre Desplat may very well be the person that scores the movie and we’re not saying he won’t be. We’re not saying he isn’t involved. But we were told there was no contract as of a month ago and there are other candidates waiting in the wings. It also should be taken into consideration that Alexandre Desplat was never formally announced as the composer for Rogue One: A Star Wars Story. My initial reaction was this was impossible and then I remembered it was never official, which is strange and gives the rumors something to at least consider.
We initially heard this confusing rumor a short time ago but knowing that the trailer was dropping, we hoped some kind of formal announcement would be made confirming Desplat and to the best of our knowledge that has not yet occurred. It could just be business as usual. Maybe they simply don’t want it to become a “thing” that John Williams isn’t doing the music? Maybe it isn’t crazy to not have contracts formalized at this stage?