The origins of a Chaos Legions codex being separate from Chaos Space Marines is a couple years old, arriving shortly after the current CSM codex was released. StraightSilver goes into some of these details below.
Please remember that these are still classified as rumors. Nothing is written in stone, take with salt.
via StraightSilver
Well I am pretty sure that the starter set will feature Dark Angels and Chaos, people I have spoken to have backed this up although it won't necessarily be Chaos Space Marines.
Both Jervis Johnson and Alessio Cavatore have both stated not only on the internet, but in the pages of White Dwarf (admittedly a long time ago for WD, but Alessio's blog post was just after he left the company) that the current Chaos Space marines Codex would remain to represent Renegades, and Chaos Legions would be in addition to the current Codex.the current CSM Codex was steered more towards Renegade Chaos Space Marines.
So I am 90% certain that there will just be one new book, but it will feature cultists as a unit for Alpha Legion in much the same way Codex:CSM3.5 did.
Apparently a Traitor Guard Codex was suggested, and considered, but later rejected so I suspect if they are coming in any shape or form it will only be in the Legions Codex.
I haven't heard anything about Chaos Daemons Codex being replaced (although it should be!), but that Legions will have better than summoned daemons.
In one of his Standard Bearer articles Jervis Johnson also talked about this. It was an article talking about the different design philosophies that had been employed in the past, and for once admitted when they had got things wrong. He said that they knew immediately after they released Codex CSM that they had got it worng, and that it would remain as a Renegade CSM Codex, but they they had started work immediately on a new Legion Codex which would address these issues. He did state in that article though that the current CSM Codex would remain, and that the Legion Codex being worked on be in addition to that.
Bear im mind though that this was a couple of years ago so may now be outdated info, but all my sources are telling me that the current CSM Codex is staying (although it may get a WD update), and that the Legions Codex will be quite different.
the Legions Codex was started quite some time ago (apparently almost immediately after the current one) and so has been in development for quite some time (although not really that long when you consider there are models started more than 2 years still awaiting release).
I actually don't see a problem with the current staying as Chaos Space marines, as in my opinion that Codex does a good job of representing Renegades.
I think the Legions Codex will be very different.
Alpha Legion will get a cultists entry.
I would hope (although this is wishlisting) that Iron Warriors would get access to Basilsisks etc agin (look at the pics in the latest WD - Chaos fighting alongside Guard with artillery support. Is this a sign of things to come?)
I would also like to see some love for Thousand Sons and Emperor's Children.
I honestly think Legions will be as different to CSMs as Grey Knights are to Ultramarines
When I spoke to my sources at GW they did say that Codexes for seperate Legions were put forward and rejected, as was a Traitor Guard Codex.
The Legions Codex apparently will have a bit of everything for everyone, but won't include details for Renegades as these are already covered in the CSM Codex, which leads me to believe it is staying.
I have been proved wrong before though so don't hold me to that, just passing on what i have heard.
That interview does also state that the current CSM Codex was written as a Renegade Codex, but then the artwork and painting section was added in and it ended up being retitled Codex Chaos Space Marines.
They intentionally left a lot of stuff out. At the time it was apparently to simplify the previous Codex, but the plan was to later release something regarding the Legions themselves.
Jervis Johnson says in his interview that as soon as the CSM Codex was released they realised they had made a mistake and set to work writing the Legions Codex almost immediately, but that was some time ago now, so who knows if that's all changed.