Before and After Achievement in Halo: The Master Chief Collection

For this achievement, all that it necessary in order to experience the game in both modes is to switch between Classic Mode and Remastered Mode once or twice for all ten missions within the Campaign. To do this, simple tap the view button on the controller and this will toggle between the two modes with nearly no delay whatsoever.

You may end up finding yourself naturally switching between modes as you explore the vast landscapes and structures of each mission. Thus, this achievement may come naturally without even thinking about it. Just keep in mind that you'll need to toggle the visuals once or twice per mission in order to make progress towards this achievement. Once all ten missions have been seen in both Classic Mode and Remastered Mode, this achievement will finally unlock.