To change your shot from a draw to a fade OR a fade to a draw, first complete Windy Valley Hole 1. Then win the Dune Club Tournament, and return and complete Windy Valley Hole 2. After doing so, the man will offer to change your shot. Even if you reply 'no', you can return and change it anytime. Note - He can only change your shot once, so your change will be permanent.
Press and hold Select as you choose your player and keep it held down until you reach the hole to play. When you get to the hole you should see that your player is left handed. This only works with Mario, Wario, Luigi or Club Champs.