The body is changing, but I am not changing. I am eternal. Therefore I am not this body. I am not changing. This simple truth, the first instruction of Bhagavad-gītā, the Kṛṣṇa has begun instructing Arjuna . . .
After Arjuna has accepted his discipleship, the first instruction is this, that "You are lamenting on the body of your grandfather and teacher regarding fighting, but your grandfather, or the soul . . ." which is already explained that, "You as soul, I am as the supreme soul, or all these kings and soldiers who have assembled, they are also souls. They existed in the past. Now they have changed their body. Again they will change their body, but they will exist." This is the instruction. The body is changing, and the vivid example? That in this life you are changing body.
So what is the difficulty of understanding this simple truth that the soul is different from the body? And so far eternity of the soul, that is also, there is proof. Because in childhood I was present, in boyhood I was present, in youthhood I was present, and in this old age I am still present.
So naturally it is concluded that when I change this body, I exist.