Some improvements to the game require us to make changes that will invalidate past saved data (mainly impacting films and campaign saves). This includes your mid-mission progress (in and out of playlists), any Theater films, and game options such as input bindings and video settings. The saved content missing may be different depending on the platform. This does not impact achievements.
As Halo: The Master Chief Collection only supports temporary Theater films, we strongly suggest that you record your favorite moments soon after generating the film, using GameDVR or your preferred recording method. Do this early to avoid loss of data during a title update.