Saying that Codex: Chaos Legions will replace Codex: Chaos Space Marines is like saying that Codex:Grey Knights would replace Codex:Space Marines.
I don't doubt that one Codex sells way more than another, but it doesn't mean that one shouldbe withdrawn.
Chaos Legions will basically address all of the stuff that is missing from the current CSM Codex, so the two will be radically different.
To say that it will repeat stuff isn't necessarily true, after all the stats for Blood Angels, Space Wolves and Ultramarines are pretty much identical, but they all have their own Codices.
I don't think people realise just how different Legionnaires are to Chaos Space marines, and this will be reflected in the new book.