It was released in early September and did incredibly well for Warner Bros., even beyond the studios wildest nightmares, er, dreams. In response, Fox cut a trailer for New Mutants that played up the scary elements from the film, essentially selling it as a straight-up horror movie this despite the fact that in its first test screening, New Mutants scored the exact same number as the first screening of Deadpool and besides the very end, reshoots were deemed unnecessary. However, because of that well-received trailer, audiences were now expecting a horror movie.
At that point, Fox decided that instead of doing the three days of additional filming required to complete the YA movie that everyone had initially agreed to go make, the studio would change course and make a new version of the movie that was straight-up horror, which was what Boone and Lee originally wanted to do in the first place. Its a little like a diner sending back a meal at a restaurant not because there was something wrong with it, but because they simply changed their minds and wanted to order a different entrée, or movie, in this case.
Those close to the director said he always felt a bit neutered during principal photography, as he was forced to tone down the violence in order to avoid a dreaded R-rating. And because Boone was contractually obligated to shoot what was in the script, there was only so much he could do despite his and Lees best instincts. Sources say that while the second half of New Mutants is basically locked at this point, the first half of the film needs some work, and that Boone will likely add a bunch of scares that didnt initially appeal to Fox execs. He and Lee might even stand a better chance of getting what they want now than they did during the initial development phase.