https://www.warhammer-community.com/2017/08/13/daemons-and-thousand-sons-in-codex-chaos-space-marines-aug-13gw-homepage-post-2/
Doesn't actually outright state it in the associated
FAQ, which is really helpful, but here's the snippet from the article:
Secondly, if you’re a Thousand Sons player, you may have noticed that the Dark Hereticus discipline in the new codex is larger than the one in the Index. Don’t worry – the discipline isn’t being replaced and you won’t need to get Codex: Chaos Space Marines to keep using it. Similarly, if you have Codex: Chaos Space Marines, you’re welcome to use the expanded Dark Hereticus powers with your Thousand Sons Psykers. If you’re a Daemons player, this change also applies to Be’lakor.