Easy, just create and destroy MASS...................
If we could create mass on a big enough scale, then maybe we leave Mars and Venus alone and just create a string of new Earths.
If we can create mass, then I assume we've advanced to where we can create any element. Streets paved of gold. Commodity metal prizes near worthless.
Of course if the goal is to modify gravity, destroying Mass sort of defeats the purpose. If you destroy the mass of Venus, then you've successfully disrupted gravity between Venus and the Sun, but there is nothing left to collide with Mars.
Either solution works. disrupt gravity or create new mass. But you have to tap into an external energy source if you go the create Mass route. Creating Mass would require a lot of energy. And if you consume all of Earth trying to create a mass the size of earth, then again, you've defeated the purpose.
I hope to walk streets paved in gold one day..................