The cost difference is the servos. The 9CAP and the 9CHP each come with four servos but the 9CHP has better ones. I think the 9CAP can be had with either 3004 or 3001 servos and the 9CHP comes with 9001s. There may be different combinations available, but these are the most common.

I recently had my 9CAP converted to a 9CHP by Hobby Services. They removed the throttle ratchet, changed the switches and faceplate. The cost was around $35 and they completed the job the same day they received my transmitter.

Eric