So, I searched for the answer to this, but the search engine sucks and only came up with one post with the words "Primaris psyker" in it. Grr.
Anyway, my question, since I can't find it in the FAQ, Codex, or BRB, is this: The FAQ says a primaris psyker can swap his codex powers for ones from the main book, but does he have to swap both of them, or can I, for example, ditch Nightshroud and keep Lightning Arc? It would seem that he'd have to swap both, but I'm not certain.
"The idea of some poor slob cowering in a trench and wetting himself as a horde of orks charges forward appeals to me." -My friend on why he plays Imperial Guard.
I can't remember where it is said, but It is per psyker, and they can either have powers from the rulebook, or powers from the codex, no mixxing and matching. However it is done per psyker, so you could have one Primaris with rulebook powers, and another with codex powers. Hope this helped -Empirespy
The Rules are different for Psykers who have random Table in their Dex (i.e. 5th Ed Dexes), and Psykers who get pre-selected Powers from their Dex (6th Ed Dexes).
Essentially, what it boils down to is that you can never choose to mix pre-selected and randomly rolled powers.
The Amendments Section of the IG FAQ does actually cover it, if not as clearly as it could. It tells you that you have to choose between Codex and Rulebook Powers, and if you use Rulebook Powers, you get two of them, no more, no less.