> Which, given as we are talking about sin and atonement, has no relevance.
I should have been more precise. It's not about atonement by means of killing something to satisfy God's wrath or something like that. It is about atonement in terms of freeing people and the rest of creation from its "bondage to decay."
> Besides, sin and death still exist so it was pretty ineffective.
They do still exist, but Christianity holds an inaugurated eschatology: while these things still exist they have already been defeated in principal though they have yet to be totally destroyed.
> That is vicarious atonement. Rewording it doesn't change the meaning, or the theology Christianity espouses
OK then I might not get how you are using that term. By vicarious atonement I would think of something like person X does something deserving punishment, person Y gets punished instead and that counts as the punishment for person X. In terms of Christian theology, the person Y would be Jesus, but that is exactly what Paul says doesn't happen. Jesus isn't condemned; sin is condemned. That's a major difference.
> So what changed?
Lots of things. See inaugurated eschatology in general but also the way Christianity offered a new way of being human in the ancient world that no one had thought of before. A lot of early church history is trying to work out how we must live given that God's new age has dawn here and now. For a good summary with lots of concrete examples of the changes see Ortberg, Who is this man.
> I'm not sure what you're getting at
I'm not really disagreeing with you. I agree that you can't draw a straight line from the Levitical sacrificial system to Christian atonement theories. I'm just saying that there is some evidence of blood being involved with individual atonement in Lev. 16. e.g. verse 11.