You will treat it "as if" this was two physically separate vehicles. First work through the existing leased vehicle and show it's disposition and be done with it. Then you'll add a 2nd vehicle which will be your purchase/acquisition of that same vehicle "as if" it was a physically separate vehicle that has nothing what-so-ever to do with the leased vehicle.
When done, you will have two vehicles listed. The old one you "disposed" of, and the new one you "acquired" after disposing of the old one.
