Energy is conserved, the total energy never changes.
At the highest point, the velocity of the pendulum is zero. That is, the kinetic energy is zero. The total energy consists of potential energy only.
E = E pot = 503J
If we set the potential energy to zero at the lowest point and the total energy can not change, all the energy must have been converted into kinetic energy.
E = E kin = 503J.
