https://www.warhammer-community.com...-questions-answered-july-5gw-homepage-post-2/Can I combine units from the index and a codex into one army?
The datasheets in the new codexes overwrite the same datasheets in the index books. You can certainly use units with updated datasheets alongside units from the index that have yet to be updated. Once a unit has been covered in the codex though, we assume you’re using the latest version.