Please see my post 435: https://www.hometheaterforum.com/community/threads/2001-a-space-odyssey-4k-uhd-available-for-preorder.356390/page-22#post-4643328 As best as can be determined by primary source interviews, work was underway at Warner for a new 4K restoration with Kubrick's longtime assistant Leon Vitali consulting on the project. The aim had been to produce both a UHD release and a new theatrical DCP which could be used as part of a 50th anniversary celebration. Nolan was in the same building at the same time working on remastering his back catalogue for UHD. Knowing of Nolan's fondness for "2001," when the 2001 team was looking at different elements for reference, they invited Nolan to join them for a screening. When he saw the 1999 interpositive, he felt that it looked so good that audiences should have a chance to see a 70mm version based on that specific element. Warner then decided to go ahead with a 70mm reissue in the spring, under the supervision of Nolan, while simultaneously continuing with their 4K restoration. The UHD was always going to happen, and the theatrical re-release in IMAX was probably always going to happen (it had been rumored for well over a year). The 70mm reissue of the "unrestored" version was the unexpected part of the equation.