everything is shot digitally now as opposed to old film. The film itself is re-scanned into individual images, and for the last Blu-ray release that is what they did. This 4k version is some sort of upscale for that I guess. I don't know how many special effects they are replacing with new, like explosions for example but the audio track is all newly remastered to use the new dolby atmospheric encoding so technically a 9.1 surround sound. This is done, all under the supervision of the director of the film I understand.
other re-releases have been simple cleanups of the last version, the original Blu-ray re-release was that. A lot of these re-releases are like that and that is how they get a bad stigma about them. the titles are usually just put on the shelf to create hype for the media in it's early stages....now that 4k is actually gaining ground, they can put work into an actual re-release.
the newest fifth element was a complete re-start to make a new 4k disc and holy crap is it nice, its astonishing the little things you see now, stuff you don't notice though if you haven't seen the movie as many times as I have
