How move from Ammo to PhysX?

Since i am getting very bad frame rate on android native build on my 3d game, i am trying to move to PhysX.
But changing the option on project settings seems not be enough.
I got a lot of:
Cannot read property ‘physicsWorld’ of undefined
Cannot read property ‘defaultMaterial’ of undefined
Cannot read property ‘SphereGeometry’ of undefined
Etc…

Switching back Ammo/Bullet everythings works.