If your body changes, then you clearly are not the same person you were just a second ago. Cells change second by second. Some die, some multiply. You are never the same second by second.
No, if my body changes, then the thing that has changed is not myself, but my body. This is why I say “my” It is my possession, but I am not my possession. I have changed my clothes many times, this does not change me as a person. I have changed my residence many times, but does this does not change me as a person.
If your “I” was your body then you would have no I-awareness. That is because the body being constantly changing would never be able be to aware of any single I-awareness. Moreover, it would mean that every cell in your body would have its own I-awareness. Then you would be multiple people not one. This is not true, it is obvious I am only one person, who is aware of a changing body. It is also obvious that the body can continue to exist after physical death, and yet have no I-awareness. This therefore means that the body cannot be the self, but rather is a possession of the self.
Likewise my mind does not have any I-awareness, otherwise everyone of my thoughts would have I-awareness and be thinking things. I would be multiple people not one. This is not true though, I am only one person, who is aware of thoughts, feelings, sensations. My thoughts, feeling and sensations have no “I-awareness” but it is I the person who is aware of them. I watch them rise and fall, and I can even control them. There is only one controller.
Change is constant. True. But for change to be constant there has to be one to be aware change has taken place and that itself would have to be unchanging. It is obvious that one cannot consistently deny the self, because even denial of self, requires one to doubt. If there is no self there is no “I”, no perceiver, no doubter, no knower, no experiecer.
There are not many things in the world that we can remain certain about because we doubt everything. We doubt things we perceive, we doubt our theories, beliefs, interpretations. But one thing we clearly cannot doubt is that “I” exist. This is not something that is open to doubt because it is not something we know, it is something self-evident. Thinking requires a thinker; acting requires an actor; speaking requires a speaker. The self is an absolute precondition to know, think, and act.
The “Self” is what we know to be consciousness because we are conscious beings not unconscious beings. Bodies, thoughts, and actions are unconscious things, the only thing we know to be conscious is the “I”. Now, knowing this is where the masterstroke comes from Hindu Vedanta philosophy, the “I” is something that is distinct from body and mind and it is the only substance that does not change. It is therefore not something within space and time. It is not born and it does not die. It is the basic “I AM” condition that is existed for eternity. The aim of Yoga is to undo this confusion we have made by mistaking the “I” to be the body and the mind.