That is not possible for many reasons, the major being that due to legislation if a model of a car does not come out in our case as 2.0l from factory, you are not allowed to register the new engine and make it road legal. So they could potentially confiscate my car if they caught me.
I'm guessing you're not in the uk then? I know we sometimes have to work around some ridiculous rules, but that particular one seems very unfair.
I seem to remember reading that some of the changes that were proposed to our MOT rules would effectively achieve a similar thing to this. i.e. car needs to be as it left the factory or any modifications need to be type approved etc - not sure what the current state of play is with regard to these proposals but it is very similar to some of the legislation in other european countries.