Is it possible to take a black leather jacket and lighten it enough to change the color to something like olive green?
Light to dark is easier, you will struggle with black. Depends how far the present finish has gone into the leather and whether you want to risk ruining it by using powerful chemicals to strip it.
How about an intro?
im Mike, 20 year Naval Flightcrew Living in Vegas. Interested in making a post-apocalyptic jacket (Mad Max) style circa 1980s