You don't change a good thing. The Super Duty has been good to Ford with the exception on the first few 6.0's. So I wouldn't look for a major redesign until the sales fall off a bit and they start loosing ground to the other truck manufactures. As Bill said the '08 were as major of a change as you will see them take when they are doing well.
Ford never could go much beyond 3 or 4 years without some kind of face lift.
Other than superficial changes to the sheet metal, the Super Duty has never really changed since it's conception in 1999 when Ford separated the pickup line into independent light duty and heavy duty chassis.