I am glad to see this implemented.
I tried it out and I have an area I plan to map regularly. I flew a large portion of the area and then two smaller portions at various heights testing accuracy and overlaps etc. When I turn on the existing base layer option, I only see the small portion of the map as the base layer (plus the outdated google map). This was the latest flight so I am assuming thats how the app is implemented.
It would be great if there was an option to select which mission was used as the base layer or if it combined them all into one current layer.