Hey all, just leaving this here as I can't find any other documentation on the refactor > replace functionality on materials in Lens Studio.
Basically, it's a convenient way to bulk edit materials on a lot of imported objects. For instance, if you want to assign a graph material to a lot of objects instead of editing the default material they are imported with, you will need to create and assign them a new material. Instead of doing this one-by-one, by going to the material that you want to replace (the one that's already used in the scene), right-clicking it and choosing "Refactor > Replace", then choosing the new material, you can replace the material en-masse.
Anyway, hopefully anyone Googling this in the future will be helped out by this post!