From the looks of it, they're basically remaking the art to fit new canon, and to modernize to the current graphical standard. There will likely be tweaks to the physics engine, to put it more in line with Halo Reach and Halo 3, and animations are likely to be redone. I doubt the voices will be redubbed, though it's possible they'll have more dialogue recorded by the original actors, for in-game chatter and the like. Sound effects will likely be updated, and it's somewhat likely the level design will be changed ever so slightly to fit the new physics capabilities and graphical aesthetic.
However, that leaves the story and overall gameplay, Halo's two greatest features, untouched.