I suppose this most recent conversation could be tied to the topic.
From what I understand, Callan wants to know how history is changed... right?
Look at the Dunyain. To them, the subjective history of the world doesn't include Sorcery/magic, though in reality it does exist. Kellhus learns of it, and suddenly history changed.
I think the confusion arose when you started talking about subjective and objective history. No, 'hisotory' doesn't change. What happened, happened. But history, or our understanding of what happened, changes over time. Take some fact that you 'know' is correct.... Like the Holocaust for example, or the Moon landing. This historic events are so defining, its nearly impossible to conceive of a time when humanity forgot. But even today you can find plenty of people who don't believe that either happened, even though people that were there are still alive. Now take an event form a few centuries ago, and you can't ever be certain that it happend. The powers that be tend to change "history" to better suit them.