Well it is possible to update a car from an older version to a newer version of Racer, most of the time. Sometimes the original author in an attempt to keep people from altering their cars have locked their model files up in a .ar file, which can make it a right pain at times. And really what you need to do mostly depends on the car.
The most obvious change is going into the car.ini and changing the version, lets say it looks like this
"version=050b8"
This would need to be changed to this, to work with the newest version of Racer.
"version=090"
The next thing to update would be the shader (car.shd), of course this depends on the car and the system you are running. If your computer runs Racer with CG shaders and the car you are updating does not run CG shaders, this will have to be changed (I'm not diving into a full explanation of CG shaders). The easiest way to identify between the two is CG will always have things like "vf_standard" or "vf_reflect" in them, and non-CG Shaders will tend to use a frensel texture
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