John Hamm was in the pilot but his role was replaced by one of the female writers.
According to Lizzy Caplan and Jon Hamm on the Tonight Show, Hamm was cast in the pilot but replaced by a woman.
Laura San Giacomo played Ann Sorelli in the original pilot. However, she was recast due to creative differences. After Kiele Sanchez was brought in as a replacement, 80% of the pilot needed to be re-shot.
Jon Hamm was replaced after the pilot episode with Dan Futterman, who is much more well known as a writer than an actor.