Older 2k movies are more easily upscalable to 4k or 8k compared to newer 2k movies because they were shot on film cameras which display more detailed images than digital cameras. Additionally, there was no computer-generated imagery back then, so artists relied on practical effects which look much better in 4k. There is also more detail to start with on a 4K DI, so theoretically, you can get a better image onto disc, but there are examples of 2K upscale as reference 4K PQ too. To upscale older movies to 4k, the old film element is run through a 4k scanner which scans each and every picture singularly, so it can take a while and it needs enormous amounts of digital storage space. However, the end result is worth it as the increased resolution can give older films a new lease of life and provide a much better experience than when the film was first released into cinemas.