If a perfect copy of you disagreed with you, which one would you accuse of changing?

Both of you would remember being the original, and both would be right by every internal measure available. There is no fact inside your own experience that could settle which one had drifted, which suggests that the sense of continuous, stable selfhood is not evidence of anything, merely a feeling that comes standard. You cannot check yourself against yourself.

Which is a reason to rely on outside references. Ask people who have known you a long time whether you have changed and in what direction, and take the answer seriously even when it does not match your inner sense. Your own certainty about who you are is not the reliable instrument it feels like.