Post-production
Boone and his regular editors Matthew Rundell and Robb Sullivan delivered a cut of the film to Fox that they were happy with, and it tested as well as initial screenings of Deadpool did. Three days of additional photography were planned to complete the "YA movie" that Boone, Lee, and Fox had agreed to make. However, following the successful release of the film It (2017), the studio cut the first trailer for The New Mutants to focus on the "scary elements from the film, essentially selling it as a straight-up horror movie". With the positive reception to the preview, Fox decided to make the film more like Boone's original vision rather than completing the version that they had been making during production.
In January 2018, the film's release date was pushed back to February 22, 2019. This allowed it to avoid Deadpool 2 (2018), which had just been moved to a date that would have had both films in theaters at the same time in certain markets. It also allowed time for the reshoots the studio required to make the film more in line with Boone's original vision. When asked about this delay the next month, Williams stated that there had been concerns during filming regarding the short turnaround from the end of photography to the previously set release date, especially with the amount of visual effects still needed to finish the film, so in her opinion the delay was "for the better". The additional photography was scheduled for mid-2018. It was expected to include the addition of several new characters who would appear throughout the film rather than just making cameo appearances.
In March 2018, Fox again delayed the film's release, distancing it from the new February 2019 release date for Dark Phoenix, rescheduling its release for August 2, 2019. At this point, the studio believed that the reshoots required were more extensive than previously considered, with intentions to reshoot at least half of the film. The intent was to make the film as distinct from the other entries in the series as possible, looking toward the variety of Deadpool and Logan (2017). They believed the film would not endure a similar fate to Fantastic Four (2015) which went through similar production issues and became a "flop", with the issues not being placed on Boone, allowing him to write and direct the reshoots to complete his original vision.
In addition to adjusting the tone of the film through reshoots, they also planned to change an antagonist subplot in the film. Originally intended to feature the Essex Corporation, which was first teased at the end of X-Men: Apocalypse, with a post-credits scene featuring the reveal of Jon Hamm as the villain Nathaniel Essex / Mister Sinister, Fox requested the reshoots to include a new post-credits scene introducing Antonio Banderas as Sunspot's father Emmanuel da Costa. Kinberg later denied that an actor was ever cast as Sinister when he revealed that the character had been intended to be included in the canceled Gambit film. Hamm later revealed that while there were talks to have him play Sinister, his scene was never filmed.
Reshoots for the film were slated to begin by the end of September 2018, with Kinberg stating: "audiences really embraced the notion of a superhero movie or a comic book movie that was, in its core, a horror film". Following the acquisition of Fox by Disney in March 2019, it was revealed that the additional photography had still not taken place, nor were they "planned so far". The work was dependent on Boone's involvement, and decisions being made to instead release the film on one of the streaming services owned by The Walt Disney Company. A month later at CinemaCon 2019, Disney confirmed that the film will be released, but indicated that its release date could be adjusted to better fit within Disney's existing schedule. A month after that, the studio pushed the film's release back to April 3, 2020, and the reshoots were set to take place later in 2019. Kinberg explained that they had take time to reschedule because the creative team was still deciding what portions of the film to reshoot due to the difficulty of reuniting the cast given their busy schedules.
In August 2019, Disney was reported to be unimpressed with the original cut of The New Mutants, believing it had "limited box office potential". Additional work was completed, after Disney acquired it, to align the film with Boone's original vision, and test screenings with these changes had been positive. In addition to making the film more horror-tinged, these changes also included removing overt connections to the X-Men films. However, the film as released does make reference to the previous entries, with the inclusion of the Essex Corporation connecting the film to the post-credits scene of Apocalypse and to Logan. By December of the same year it was confirmed that work on the film had been taking place, while marketing had been adjusted to align with the style of Marvel Studios. Boone stated that the latest version of the film followed his original vision. In January 2020, Disney's official fan club website D23 described the film as a "new addition to the Marvel Cinematic Universe (MCU)", a statement that was quickly picked up by fans and reporters. Soon after, all mentions of the film were removed from the website, while Disney identified this as an error and stated that the film would not be part of the MCU.
On March 7, 2020, Boone stated that the film was complete. Shortly after, he explained that work on the film had halted when Disney's acquisition of Fox had begun and so no reshoots ever took place on the film, not even standard pickups that had already been scheduled during initial production. At the time, around 75 percent of the film had been edited while much of the film's visual effects were also not finished. By the time the acquisition was completed, Boone had moved on and was about to begin work on a new television series The Stand (2020–2021). Before he started production on that series, Disney asked Boone if he would return to finish the film. Rundell and Sullivan were committed to working on The Stand at that point and could not continue editing The New Mutants, so Boone brought on editor Andrew Buckland to help finish the film.
The work required to finish the film when Boone returned involved completing the visual effects and editing alongside co-writer Lee, which took several months. Reshoots for the film could have been scheduled at that point, but Boone found that the three-year wait since principal photography meant that the young cast had aged too much. He also felt it did not make sense to add the post-credit scenes of Banderas since it was unlikely that they would be able to make a sequel now that Disney owned the X-Men rights and was integrating the property into the MCU. Describing returning to the film after so long, Boone said, "we hadn't seen it in a year. We did a bunch of things here and there that we hadn't thought about or noticed a year before." The visual effects that still needed to be finished included Illyana's sorceress abilities, including her Soulsword that she materializes, as well as her dragon companion Lockheed. Visual effects for the film were provided by DNEG, Method Studios, and Moving Picture Company, with Olivier Dumont serving as VFX supervisor. After the film was completed, Boone reunited the cast for the first official screening in New York City, after which Williams stated, "The movie is exactly the movie we set out to make."
Disney removed The New Mutants from its release schedule, along with several other films, on March 12, 2020, due to the COVID-19 pandemic, and was looking to reschedule the film's release to a later 2020 date. On May 4, the film was automatically listed for home media pre-order on Amazon based on the film's previous April 2020 release date. Amazon took the listing down hours later after it had been widely reported on. At that time, the film was still expected to receive a theatrical release rather than be released straight-to-streaming as other films had been during the pandemic. Shortly after, Disney scheduled the film for release on August 28, 2020.
