In a way it is "re-evaluating" the entire history for every token it generates.
So yes, I know that I can just do a simple text replacement somewhere in the history but that's not really useful. I want the conversation to be re-evaluated from that point because it might have gone differently with that changed message.
The way this is done, technically of course, is you build the "Context" by walking back up the tree parent by parent, until you reach the root, building up 'context' as you go along, in reverse order. That will then always be the 'right' context, based on branching.