The few computer simulations that anyone knows how to make of the formative years of planet formation suggest that the planets did form very close to their current locations to within, say, a few 10s of millions of miles or so.The gravitational mass of the entire solar nebula was rapidly changing during this time, and large amounts of mass were lost from the inner solar system. This affects the value of 'M' which produces the gravity that a planet feels in its orbit, and so orbits changed quite a bit early on. Also, impacts with other planetoids also had their effects too.