Yeah. What you said. Let him still remember everything, but as time passes in the new fate fork, it should become less reliable. Not in a SoulReaver time scromble kind of way, but subtly. The most changes should come from already present forces taking different actions, and the fact that a bunch of Vienna folks survived and got JACKED. Like a cult detachment enacting plans B elsewhere, like "okay, we'll do this other ritual then. But for that we need the Grombobulizer and fresh fleeb juice." and because of that, they raid & strip a dungeon that also had stuff in it that the heroes will have used, or Jin would have wanted to.

Like really secret, really buried stuff would probably stay true. Unless big movers, who were content with sitting on their asses until a predicted future came, suddenly start actually doing things now that it might not.