The description says NTR content should be added in the Chantelle (Darnell's sister) update. From when MC eavesdropped the principal we know that the antagonists want to corrupt her and that their substance doesn't work on those women you corrupted. My theory is that you will be able to choose whether you want to corrupt Chantelle or watch how she is being corrupted by THEM, maybe compete with the antagonists over to whom she finally submits. It's also supposed to be optional and easily avoidable so I guess it will be same as with the trap content now and the game explicitly asks you whether you want to go through it.
So about the NTR content, its going to be introduced IN the Chantelle update, but as far as I know, the plan was always for that stuff to be in its own self-contained route, with big flashing neon-red warning signs all around it (klaxons too, because who doesn't love a good klaxon), that you enter near the BEGINNING of the game. Like, when you receive a call from the doctor and choose whether or not to tell him about the voice in your head kinda early.
There shouldn't be any overlap later in the story, especially given how the MC and his other personality clearly don't like to share. So to answer the other guys question, yeah, the story will change dramatically, by necessity, because the MC wouldn't undergo the character development needed to be proactive about changing his life and getting the things he wants. And apparently this town is filled with assholes who take whatever they want, so.......