“I wonder if I’ve changed in the night?” asks Alice. “Let me think: was I the same when I got up this morning? I almost think I can remember feeling a little different. But if I’m not the same, the next question is, ‘who in the world am I?’ Ah, that’s the great puzzle!”
-from Alice in Wonderland, by Lewis Carrol
An excellent question, not usually answerable by the individual asking it. For anyone to easily notice change, they must not be there to witness the change. This is why a person is still the same person, even if they have changed. For those who witness the changes as they occur, it is simply the same person growing, expanding themselves physically, mentally, or spiritually. The events until that moment had defined who they had been, and this was merely another event defining them, an event whose outcome was determined by those previous events and changes.
To any who have not witnessed the changes, however, it is quite obvious. Certainly, this must be a different person than before! Surely this cannot be the same person, for they have changed so much! This is why the other half of the answer is no, the changed person is not the same person. Anyone who does not personally witness the changes in a person as they go along cannot understand what led them to make certain decisions, what led them to become so different. If one does not witness, for example, the person tossing and turning in the night, unable to sleep, they cannot fully understand why that sleepless person becomes irritated at the slightest things the next day.
So now we are at an impasse. Is the person the same person? Yes and no. The answer is both, neither, and everything in between. Looking back on old pictures and memories, even the person asking themselves this question finds that they have changed drastically. They seem like an entirely different person, and yet they can remember the choices, decisions, and actions that lead them from the past to the present. They know why they have changed, why they could not remain the same. Looking back they see that no matter what they did at that time, they would have changed. They see that what they did was a direct result of who they were at that time. So if the changes were brought about by who they were, then they cannot be a different person. At the same time, however, they are most certainly not the same person they were.
So here the answer arrives. Yes, the person is a very different person than they used to be. No, they are not a different person for the changes were brought about by the person they were at that time. Yes and no, both and neither. This is the nature of humans. This is the nature of change.