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It's not a guaranteed method, but there is a pattern to the class grafts.
+2 to one saving throw, at _ CR they get one of the classes' iconic _ level abilities, at certain CR's they get one of the abilities replaced by an archetype, etc.
Of note is that the saving throw bonuses are only to adjust saves to closer mimic the classes' base saves.
For example, Soldiers usually have Good Fort and Will, but the Combatant Array has Good Fort and Ref. Mechanics get Good Fort and Ref, but the Expert Array only gets good Will. Envoys get Good Ref and Will while Expert Array already has Good Will. Same with Mystic and the Spellcaster Array.
Soldiers get +2 Will and -2 Ref, Mechanics get +2 Fort, +2 Ref, and -2 Will, Envoys just get +2 Ref, and Mystics get nothing.
Based on this, Biohackers (Expert) would get +2 Fort and -2 Will, Vanguards (Combatant) would get nothing, and Witchwarpers (Spellcaster) would get +2 Ref a -2 Will.
I can't speak for how I would do it myself, I'm not confident enough with the rules. However I believe on the Alien Archive 4 page in the discussion thread it was confirmed that the new class grafts will be in there.