In this context, he's = he has. So he's changed = he has changed. In a different context he's could mean he is. For example - he's Russian = he is Russian. The difference between "he changed" and "he has changed" is that "he has" hints that the change happened recently. "He changed" hints that the time of change is unknown. By the way you mentioned the other use of "has" like he has three cats. In this context he's cannot be used :D Hope that helps!