I finally got a chance to get out and drive the new Midvalley Highway yesterday, and I can confirm that the truncation of SR 138 has been carried out in terms of signage on the ground. All SR 138 shields have been removed from the road east of the Midvalley junction, and these signs have now been put up:
Here's the catch, though... where do you put the 138-179 transition point. Online highway reference puts it at Parachute Park Lane (the short road connecting Midvalley to the old 138 alignment at the east end), but those signs above are posted at the intersection with Sheep Lane.