In Rogue One we see Red and Gold Leader, Garett Edwards said that this was unused footage from Episode IV. So if they can make old, grainy, film from 1977 seamlessly blend into a major 2016 blockbuster with no one noticing
To be clear: the ANH footage in Rogue One is very noticeable. Overuse of DNR, not the same print quality. It took me out of the movie each time. It was a cool idea (and it somehow makes Lucas an additional director of R1 by the way 😃) but it was not seamless at all.