As 343 have said a several thousand times, they want the games to play exactly how they played when they were released.
Halo 2: Anniversary:
New Engine, designed for Xbox One New Graphics Updates (Weapons and Maps have new textures. Currently using some Halo 4 placeholders) New U.I. (Currently some Halo 4 placeholders) Only 6 Maps; they have been re-imagined by Max Hoberman and the Certain Affinity team Ascension: Some of the routes have been modified, a dynamic center-map energy shield can be triggered from the three points on the map.
I wish they'd tweak the campaigns enough to make them current. I never liked that sniper rounds bounce off Banshees in the first game, you can't pick up FRGs/swords, etc. It still annoys me they went to all the trouble they did with Halo CE Anniversary but they didn't tie the campaign to Reach's commendations/stats like the multiplayer.
Xbox One dis-Kinected 6/9/14. Let that be a lesson to people. Vote with your wallet. http://viper.shadowflareindustries.com/images/myroom/DSCN0220.JPG
Topic, figured they would improve the engine in the older ones
OMFG kid stop posting on things you know nothing about... first of all Halo:CEA has a new engine, it got it when it was released it's also getting it's original MP added back into it as it was using Reach's MP when it was released... Halo 2A is getting a new engine for the Halo:MCC, Halo 2s MP hasn't been playable online since MS shut down the original xbox live servers. Halo 3, and Halo 4 are getting HD textures and touched up for HD I believe other then that they aren't having anything else done to them.