u/EmotionalAd1029

1. Your brain can change, all of the molecules of your brain can change. Nevertheless the conscious sense of "I" remains the same. Your body can change, your personality can change, but you are still you.
2. When talking about your brain, you don't refer to it as "myself or I". You say "MY brain...". It is something that belongs to you, but it's not you ( your sense of awareness / consciousness ).
3. "Substances can change your conscious experience". Yes, the experience of your brain, but not the awareness beyond the brain. Try meditating while being drunk. You'll notice that the observer remains the same no matter if you are drunk or in a clear state of mind.

These are only my thoughts. I don't claim that they have necessary any scientific or philosophical value.

Cheers.