Hi Thehornet,
No need to multiply posts on the same subject...
To give you a quick answer :
Basicaly you can't. Because Texture mapping (the way texture will be applied to the car) is obviously not the same on an old (low-poly) car and onto a new (high-poly) one.
So most of the time you have to start your texture again from the car mesh projection and fully repaint it.
It's not just a decal you can copy from an old car and paste on a new one the way GP3 was doing it as all cars where using the same 3D shape.
Regards