What do we know about cars in the Cars universe? Well, for starters...
They're living creatures, living as in organic.
Q. In Cars 2, you mention the existence of dinosaurs. Are those dinosaurs
flesh and blood or are they cars? And further, do you have to vet
those sorts of references that may introduce a different aspect to the
JL: I mean, in the mythology of the Cars world, the living creatures are
vehicles of some kind, and so it’s like with dinosaurs. I always
imagined it would be some kind of dinosaur-like car or vehicle,
something like that. It would actually be an animal. And whether it
would actually go down in the earth’s crust and get smushed and
actually make a fossil fuel, I don’t know — we don’t go kind of that
deep into the mythology of the world. - John Lasseter
Can a car's engine be upgraded?
Certainly they can. In Cars 2, Sir Miles allegedly guts his existing (diesel) engine and replaces it with a fancy electric one, powered by replacing his windows and roof with solar panels. No-one seems to think this is especially mind-blowing, albeit some people think he's a hippy for doing it.
Can a car's other (mechanical) parts be upgraded?
Yes, the parts that are most obviously replaceable are exterior elements; bumpers, engines, headlights, eyes, exhaust systems, etc etc. The only parts that seem integral are the bits inside the cab which presumably is where the organics are found.
So basically it's only the internal organs that can't be upgraded?
This is the nub. The filmmakers apparently gave some serious thought to where a car's brains and other organs are located, before ultimately giving up and settling for a strict rule of funny interpretation.
Interpretation #1 - The brains are found inside the driver's cab
Interpretation #2 - The engine is the brain
So are cars immortal?
No. We see multiple old cars, some of whom are very wealthy. If it was possible to do a full upgrade, there's no reason to assume that they wouldn't. Since the internal organs are, presumably prone to the same sort of aging as humans suffer then simply upgrading your machine elements wouldn't make you immortal.