The recent reshoots are behind the switch, according to sources.

Rogue One underwent extensive reshoots this summer that saw writer Tony Gilroy take on a filmmaker role during the shoot and the postproduction process as Disney and Lucasfilm sought to clarify story and tonal issues for the pic that is set to open Dec. 16.

But the reshoots also altered the scoring calendar and Desplat, who won an Oscar for The Grand Budapest Hotel, was no longer available. Disney and Lucasfilm then turned to Giacchino, who has a long relationship with the studio and its various divisions.

For Pixar, the maestro created the music for Incredibles, Ratatouille, Up (for which he won an Oscar), Cars 2 and Inside Out. For its live-action film division, he has worked on Sky High, John Carter and Tomorrowland. For Disney Animation this year, Giacchino scored its surprise hit Zootopia. And he’s making his Marvel Studios debut with Doctor Strange, which opens Nov. 4. (With Rogue One, he will end up working for all four of Disney's film arms.)

Giacchino also is a Star Wars fan and appeared as a storm trooper in Star Wars: The Force Awakens, directed by his pal J.J. Abrams and for whom he has scored films and television shows.

Giacchino will now become the first composer to work on a Star Wars movie other than the film series' iconic mainstay, John Williams, who is slated to return for Star Wars: Episode VIII.

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