Avengers: Endgame reshoots changed the rules for how the film dealt with time travel. Considering how big of a role time travel plays in the film, it was especially necessary that Avengers: Endgame's creative team got it right.
In Avengers: Endgame, the surviving Avengers post-Thanos' snap discover that there is one solution to reversing the catastrophic damage that was done in Avengers: Infinity War. With Hank Pym's technology, they discover that they can travel through time and space in order to retrieve the Infinity Stones. It turns out to be more difficult than they might have expected (among other things, running into their past selves ends up being a notable hurdle), and even Tilda Swinton's Ancient One from Doctor Strange makes an appearance to inform Bruce Banner/Hulk that tampering with time is not to be taken lightly. As it so happens, though, her appearance in Avengers: Endgame was involved in the film's reshoots, resulting in some modifications to the way time travel was portrayed.
Speaking with IndieWire, Swinton revealed that she "went back to reshoot [the scene] because a couple of lines had been changed, a couple of plot points had been changed." She then elaborated on the changes, explaining that they included the visual timeline her character shows to the Hulk, addressing the major consequences the universe would face if the Avengers failed to put the Stones back where they found them.
Considering the obvious complications tied to time travel, even the writers and directors failed to agree on how it would work in the film. That said, the reshoots allowed them to create a more concrete understanding of how it'd work in the MCU at the very least. In fact, Swinton also revealed that Avengers: Endgame knocking Back to the Future's time travel logic was also added during the reshoots. The addition came as a result of test audience confusion, and the Back to the Future reference was added to clarify some of the MCU's rules.
"Time travel" and "clarity" don't exactly go hand-in-hand, as most films featuring time travel can likely attest to. Still, the time travel in Avengers: Endgame was, at the very least, digestible, and the Ancient One's visual explanation from the reshoots certainly helped paint a clearer picture for what sort of repercussions time-tampering might end up having on the future of the MCU, which will no doubt be explored in the future - like the Disney+ Loki series, for example.
Source: IndieWire
