Hi,
You can continue existing comprehensive insurance on an older car. I'm not aware of any definite ban - but of course it gets to be more expensive than it's worth.

However, you cannot change from 3rd party back to comprensive. The company would have to make a full inspection just to make sure you are not trying to insure an already dented vehicle.

They could - but they don't.

So when buying near-new cars, one of the first questions is - Insurance type?

And - ask to see the documentation.