343 Industries confirmed that players' file shares will be moved from Halo 3, 4 and Reach to Halo: The Master Chief Collection recently and now the developer has set a deadline for players wanting to transfer their Player File Shares to keep their player-created maps and game types. The deadline (via Dualshockers) is set to August 12 for game types and August 26 for game modes and you'll only have one chance to do so, because of "technical limitations" on 343's part.
Make sure you save everything you want to take with you in file share before then.
Do you still play Halo: The Master Chief Collection?