Why was Jim lovell bumped from Apollo 14 to Apollo 13?

Jim Lovell was bumped from the Apollo 14 mission to serve as the backup commander for Apollo 13 due to a medical issue involving the original commander, Alan Shepard. Shepard developed an inner ear condition that affected his ability to fly in space, so Lovell took his place on the Apollo 13 mission alongside Fred Haise and Jack Swigert.