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Well I disagree on the the whole they wouldn't be Soritas anymore. They are trained from birth and heavily indoctrinated, to the point that I don't think they know how to be any different. Much like the Word Bearers and how they say people prone to extremism when broken of their original faith tend not to simply scale back their beliefs, but take the opposite position. Plus, for one Acts of Faith are a lot easier for Chaos practitioners, we call it a Tuesday, secondly there's the military structure/training, and third... ...I'm just trying to explain something that has happened already. However, I still want to actually do it well.
The fact that Sisters of Battle are trained since birth and heavily indoctrinated to be completely loyal to their cause makes me think that we should do the same with the populations of Daemon Worlds. Not just keep them as slaves, but train them to fight for Chaos and give them the same discipline our loyalist brothers have. Discipline is a rare thing among the Chaos legions.
Not always simce birth. The schola progenium decides who does what after a few years, some find faith during service, or an inquisitor can recommend them to join a convent. Same of chaos marines, only those chosen by their masters can become Fallen Astartes, most don't even have functional geneseeds to give out. Gotta steal those like the cavengers they are.