I think history needs to 'freely unfold' from the point where you're trying to make your observations. Up to that point, you can just include memories. There's

I think history needs to 'freely unfold' from the point where you're trying to make your observations. Up to that point, you can just include memories. There's a question on where to get those memories, though. Do you run simulations until you find a history that matches 'close enough' to actual history and just assume the memories formed there are good? (It's not even just memories. The placement of a rock, or the sting of an insect could have a big effect or none at all. And all that information is lost to the past.) So, in the Lincoln assassination example, you could start the simulation at the moment you 'change history' because everything else that informs that moment is either present in your simulation (e.g., the temperature of the theater), or is in the past so actions on those items is informed by the participants memories. After that moment, I think you do need to simulate the whole thing. The reason is that the experiencing self makes decisions in the moment, and those decisions may inform the remembering self later. And, of course, the whole thing rests on the nature of consciousness. If it's not a black box to the simulators they might be able to take shortcuts; or they may not have reason to generate consciousness at all if they know what consciousness will do in a given scenario.