I don't know if there's some automated way but the task seems to need interaction anyway. The best might be to write a comment to each answer and write that the question has been reduced to one question now, that some parts of the answer is off-topic and provide links to the other parts of the original question so that the authors may move their answer there.

If this is something that needs to been often it might make sense to put together a good pro-forma comment.

I don't know if there's some automated way but the task seems to need interaction anyway. The best might be to write a comment to each answer and write that the question has been reduced to one question now, that some parts of the answer is off-topic and provide links to the other parts of the original question so that the authors may move their answer there.

If this is something that needs to been often it might make sense to put together a good pro-forma comment.