You can replace all of the mp4 files located in the /nativeNX/movie/. Although there doesn’t seem to be any checks for filesize, I assume you are able to fit in any sized mp4 file within that directory. Regardless, here is the full list of all of the mp4 files within MHGU, with their file names and rough descriptions.
Description of CG/Video Labelling
Movie00.mp4 - MHX introduction cinematic.
Movie01.mp4 - MHX Ending cinematic
Movie02.mp4 - meeting of Glavenus’ introduction cinematic.
Movie03.mp4 - meeting of Astalos’ introduction cinematic.
Movie04.mp4 - Mitsuzune introduction cinematic.
Movie05.mp4 - meeting of Gammoth’s introduction cinematic.
Movie06.mp4 - MHGU introduction cinematic that plays when you boot up the game.