I'm a bit confused by your post.

I am assuming you've successfully booked ORD > CDG with your miles, but you've now found redemption availability from SDF and you want to change the booking ?

BA Redemption bookings are fully flexible, so you can cancel, have the miles redeposited and then rebook for a small fee. This is probably your best course of action.

Be aware that if you booked some time ago and the surcharge you paid has since gone up, the new rate will be applied to your booking so you should take that into account.

Hope that helps.

BAH