The change in direction itself will not effect us at all. To achieve this, you just rotate the system in your head by 180°. There is no global "up" or "down" in the universe, so in free space the direction of rotation is a matter of convention.
If, however, you wanted to physically change the direction of rotation/orbit, there are two cases to consider:
If you were to very quickly (or almost instantaneously) change the direction of motion, we would die - everything that is not "fixed" on earth would continue moving on its original trajectory.
If the change in motion would be very slow, i.e. the angular velocity would slowly decrease to zero and then increase in the opposite direction, this would be different. We would not smash into the next wall, because the forces acting on us would be small enough for us to "counteract". Days would get longer at first, then shorter again, but the sun would rise on the opposite side of the horizon. For the motion of earth around the sun, there is something else to consider:
On a circular orbit the centripetal force $F_Z$ is equal to the gravitational force $F_G$, so we have $F_Z-F_G = 0$. Since $F\propto v^2$, reducing the velocity of earth will result in a negative net force, so earth would spiral in towards the sun. So if we decelerate too slowly, earth might get too close to the sun, again leading to death and doom.