Halo: CE Anniversary is using 3 engines
So that�s two engines in the single-player campaign � the new graphics engine and the original Halo: CE engine � and one for the multiplayer � the Reach engine

The coolest addition though is definitely being able to switch between the new impressive HD visuals and the old skool 2001 visuals at the touch of a button
Gameplay
� absolutely nothing, it should handle exactly the same as it did 10 years ago, and that means that even all the glitches and bugs from Halo: CE won�t be fixed. Not because they�re lazy, but because 343 didn�t want to mess with anything in terms of gameplay that could change the experience even one iota

Another new feature from a franchise perspective is the inclusion of Terminals, but unlike Terminals from iterations gone by, the Anniversary rewards will take the form of cool little videos that you can feast on. The Terminals will be more accessible and easier to find than in Halo 3 according to 343, and they will be used to tell the story of the Halo ring. If you find them all, 343 hinted that they�ll use them to lay down the foundations for the recently announced Halo 4.

Multiplayer
the multiplayer aspect of the Anniversary experience will consist of 7 fan-favourite maps from the original title, rebuilt using the Halo: Reach engine in stunning HD with a few interesting and visually impressive tweaks on how they�d originally look
E3 2011: Halo: CE Anniversary Preview ? 10 Years On & Still As Epic - Xbox 360 News At Xbox360Achievements.org