As someone who studied the lineart extensively back in mid-90s when Plus and 7 came out (and also from my many discussions with Yamato, back when I was friends with the CEO), from what I remember, it's pretty much nearly every part of the VF-19Kai/F/S/P that is different from the YF-19. You cannot reuse the same molds/tooling.
From what I remember, it's maybe only the hands, biceps and feet that are similar (and the gunpod for the F, S & P). Every other part requires completely different tooling.
Basically, there is very little comonality between the YF-19 and VF-19Kai/F/S/P.