What would happen to the earth if this happened?

You wouldn't be able to travel to the other hemisphere. Considering the speed at which the Earth rotates, and the other hemisphere rotating at, presumably, the same speed in the other direction, you would be killed trying to step across. I'm guessing you would be torn in half because half your body would be moving in one direction and half in the other. If you were actually able to get across in one piece, there would be no way to know where you would land. There is about a 70% chance it would be in the ocean.