Apparently he was sent off for violent conduct, not a professional foul as I thought. In that case the referee needs to admit an error for the red card to be rescinded. Mark Halsey stood by his decision, which should have put an end to it, given that Terry rugby tackled Jo to the ground to stop him getting through.
However, Chelsea appealed to the FA, who decided to change the rules, and have demoted to Halsey to league two this weekend. I wouldn't mind if there was some consistency, but the FA make a habit of picking and chosing rules. How can they demand players and fans respect referees if the FA doesn't?