Interestingly, the reason that Williams has had to bow out is that he’s also working on another Spielberg film, The Papers.
The only movies Steven Spielberg has ever directed with music not done by John Williams are The Color Purple and Bridge of Spies. however, according to Variety, Ready Player One will now become the third film on that list.
Due to the fact that the two movies will actually be going through post-production at the same time, Williams can’t do both, and so Alan Silvestri has been tapped to take over.