For a little clarity: These reissues will be the first time Tor has published any of the first three Halo novels (Fall of Reach, The Flood, First Strike). As such, they are making an effort to bring the visual ID of each of the books more closely inline with the titles they have published (Ghosts of Onyx, Contact Harvest, The Cole Protocol, Halo Evolutions) in an effort to give the Halo library a consistant visual presence (hence the new cover art and cover design).
In addition, Tor has gone through each of the books to clean out any small missteps that may have slipped through in previous printings (typos,errors in punctuation, etc.), which address the edits.
As for "new content?" The original story has NOT been altered. If that were the case "new content" would have been listed as "rewritten," which is not the case.
These reissues will be a great way for those unfamiliar with Halo fiction to dive into this incredibly rich universe, while offering those fans who wish to add these reissues to their collection a little something new by providing some additional value, both visually and in terms of fictionally relevant content, to the books.