With the movie already acting as a sequel to the original, producer Ryan Turek reveals how The Exorcist: Believer is retconning previous installments. The upcoming movie will pick up 50 years after the horror genre classic, seeing two families desperately looking for a way to save their daughters from the malevolent figure possessing them, turning to Ellen Burstyn's Chris MacNeil for help. Co-written and directed by the recent Halloween trilogy helmer David Gordon Green, the star-studded cast for The Exorcist: Believer also includes Leslie Odom Jr., Ann Dowd, Jennifer Nettles, Lidya Jewett, Olivia Marcum and Norbert Leo Butz.
https://screenrant.com/exorcist-believer-franchise-retcons-producer-explain/