theDarkGeneral:
Considering the broad depth of background and story line change (and expanse) that has recently hit most of the current new codexes, I wouldn't be all that surprised to see some alterations of the Chaos Legions and how they currently function, keeping them quite different from each other.
As I've stated (about a year or so ago), GW understands the need, desire, and profit margin that producing multiple Chaos Legions Codexes will bring and fulfill. Quite a few new Black Library books have been appearing, more and more on the Chaos Legions (though us World Eaters still have as yet none dedicated to us!
) all for which is to be timed several BIG changes come out next year. Many of the 6th Edition Rumors are true, and 40K will change much like Fantasy has from 7th to 8th...
From what i'm still told, there are 3 FOR SURE Chaos Codexes coming out, of which 3 Legions will appear in each, and 2 more possible Codexes within the next couple of years for all to see the shelves. Probably 2 Legions Codexes this coming year (2012), and the 3rd arriving early the following year (2013), with (possible) follow up Codexes of The Lost and Damned (named something else) and a Campaign Codex (ya'll remember the Eye of Terror?) that will be largely if not entirely Daemon based...
As for the Daemons in each, they'll largely (not completely) remain the same in each up coming Codex as they appear in the current Chaos Daemons Codex, with most getting points cost hikes and smaller maximum unit sizes, to reflect their ability to be summoned (changing in new 6th Edition) and either Move, or Shoot, or Assault...
Again peeps, just what i've been told in casual gamer talk with my GW higher ups friends...