Untill the changed gravity pulls the earth more close or further away from the sun, than we will like. Seriously, the circeling of the planets are interconnected and any big change to that, will have effects on earths year length and distance to the sun. But on the other hand, if we would have the tech to "move" mercury around, we probably could also adjust earths rotation. In either way, this is nothing remotely realistically with current state of the art.