There has been quite a few Chaos FAQ's on the GW site and now(OK, it's been there a for a while) there is a new. This version (4.0) is dedicated to dealing with how the codex should be read in context of the new rulebook.
There is no Codex Chaos Errata.
Is this FAQ the only one valid or are the older versions still valid unless they contradict any of the newest one?
The reason for this question is the lack of mentioning of the "tzeentch specific powers" that used to exist. This made it possible for a Chosen or Possessed to use Bolt of Change (and Twisting Path) without upgrading to Champion. This is not in the present FAQ, is this then not a valid option any longer?
It says nothing on the altered toughness of Obliterators, nor about the special movement modes and Berzerker glaive.