So Kṛṣṇa is giving instruction in the Bhagavad-gītā that "I" within the body is there. And the "I," or the spirit soul, that is changing the body. Dehino 'smin yathā dehe (BG 2.13). How changing? Just like a baby. A baby grows to become a child, a child grows to become a youth. Boy, a boy grows to become youth, a youth grows to become old man. So this change is not of that "I." It is a change of the outward body, which is known as shirt and coat.