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1) If it's like AoS, new factions will get proper codecies as they bring them out, while old factions (current factions) are left behind gradually.

2) They update each of the factions with a new figure range and codex to further their rules, and bringing out a couple of new ones along the way (like the Primaris and Death Guard). Possibly some of the less popular ones get the axe, like Tomb Kings and Bretonnians in AoS did.

Those are the two options I see, and it's hard to tell which could go down. In AoS, we mostly see new factions being announced and released, and they're the ones who get the new specific army books, while the older factions aren't updated, as the new ones essentially replace them in a way, while leaving the older ones in there if people have a big collection. Similar to Primaris Marines and vanilla Marines: it's a new faction based off an old one, but with updated figures and an army book.
Who knows if that'll be the case in 40k, just my predictions.]]> http://www.dakkadakka.com/dakkaforum/posts/preList/727553/9406570.page http://www.dakkadakka.com/dakkaforum/posts/preList/727553/9406570.page GMT http://www.dakkadakka.com/dakkaforum/posts/preList/727553/9406605.page http://www.dakkadakka.com/dakkaforum/posts/preList/727553/9406605.page GMT SoB so...no.]]> http://www.dakkadakka.com/dakkaforum/posts/preList/727553/9406607.page http://www.dakkadakka.com/dakkaforum/posts/preList/727553/9406607.page GMT http://www.dakkadakka.com/dakkaforum/posts/preList/727553/9406630.page http://www.dakkadakka.com/dakkaforum/posts/preList/727553/9406630.page GMT