If your old truck was transubstantiated into a Cadillac Escalade it would continue to perform like an old truck though. In a very loose sense it’s like being born again, there’s no physical change in the person but there is a very real change. I repeat, in a VERY loose sense.
Of course it would...Just like eating a cracker would perform like an eaten cracker...
And transubstantiating a truck or a cracker is by no means comparable to being 'born again'...
The change in “substance” can be explained to non-Catholics in this way:
You were a substantially different person before than after you were born again. I think you would agree that in a large and real sense ‘you are not the same person’ as before you were saved. You are changed.
Yet, if there were a full medical laboratory available at that instance of change, none of their instruments, no examination of your organs, cells, molecules or atoms, could detect that change.
What those instruments can see and detect, we call “accidents,” what changed in you, we call “substance.”