The one-time transfer will allow you to keep legacy maps and game types.
Despite a very rough road, Halo: The Master Chief Collection has become the place for the classic entries in the longrunning franchise. Even games that don’t feature the titular character have been added and most of the technical aspects have been ironed out. It’s about to get even fuller, too, as a one-time file transfer will allow people to bring over their old maps and game types from previous entries in the game.
As detailed by 343i on the Halo Waypoint blog, you will have a one-time chance this month to copy over game types and maps from your Halo 3, 4, and Reach Xbox 360 files to the Xbox One and PC versions of The Master Chief Collection. Game type uploads will occur on August 12th, and maps on the 26th. You can see a step by step breakdown of what to do at the link. This will be a one-time thing, so this will be your only chance to transfer these files.
Halo: The Master Chief Collection is available now on Xbox One, and is coming to PC as well. The next installment in the franchise, Halo Infinite, will release on both platforms in 2020.