Cowboy is correct. The shoe is on the other foot means that the position is reversed. The metaphor goes a little further, however. If you put the right shoe on the left foot, it is uncomfortable. So, the final position after the reversal of fortune is not equal, it is worse for the person experiencing it..
For example, the person who was your boss loses his position.You are the new boss and he is now your employee. The shoe is on the other foot. The situation is not only reversed, it is made more uncomfortable for him by the fact that you both remember when he was the boss.
Long time no see, Koria!