Signs of trouble were there early on. Production began in December 2018 and wrapped in April. That's a tight postproduction schedule for a film featuring extensive and complex visual effects and a full cast rendered as performance capture-based, digital, fur-covered humanoid felines who perform dance moves in environments that combine live-action and digital sets.
When the first trailer arrived July 18, the backlash was immediate. Sources tell The Hollywood Reporter that there was some redesign work at this point, including on James Corden's Bustopher Jones, who was given a more "comedic" look with a bigger mustache and longer hair. In the time leading to the release, sources say VFX work was behind schedule, the sound mix was starting late — and Hooper himself admitted that the movie was finished less than 48 hours before its London premiere.
Meanwhile, Hooper, described as a very exacting director, kept adjusting the effects. "There were no content changes, I don't think," one source tells THR, explaining that the additional week of work was about "refining, mostly small things. … refining the amount of the actor's performance you might see, refining lighting, refining integration." This was the new version Universal sent to theaters. (The studio did not respond to a request for comment.)
The VFX for Cats was handled by two Technicolor-owned VFX houses, MPC (led by its London headquarters) and Mill Film (primarily out of its outposts in Montreal and Adelaide, Australia), along with work out of additional bases of these companies, including those in India and the recently shuttered MPC Vancouver.
A leading VFX house, MPC also created the visual effects/animation for Jon Favreau's July Disney hit The Lion King, and multiple sources say members of that VFX team, including Lion King's MPC VFX supervisor Adam Valdez, were brought in to help finish Cats after Lion King wrapped.