Change of body by the atomic individual soul is an accepted fact. Even the modern scientists who do not believe in the existence of the soul, but at the same time cannot explain the source of energy from the heart, have to accept continuous changes of body which appear from childhood to boyhood and from boyhood to youth and again from youth to old age. From old age, the change is transferred to another body. This has already been explained in a previous verse (2.13).
Transference of the atomic individual soul to another body is made possible by the grace of the Supersoul. The Supersoul fulfills the desire of the atomic soul as one friend fulfills the desire of another. The Vedas, like the Mundaka Upanishad, as well as the Svetasvatara Upanishad, compare the soul and the Supersoul to two friendly birds sitting on the same tree. One of the birds (the individual atomic soul) is eating the fruit of the tree, and the other bird (Krishna) is simply watching His friend. Of these two birds — although they are the same in quality — one is captivated by the fruits of the material tree, while the other is simply witnessing the activities of His friend. Krishna is the witnessing bird, and Arjuna is the eating bird. Although they are friends, one is still the master and the other is the servant. Forgetfulness of this relationship by the atomic soul is the cause of one’s changing his position from one tree to another, or from one body to another. The jiva soul is struggling very hard on the tree of the material body, but as soon as he agrees to accept the other bird as the supreme spiritual master — as Arjuna agreed to do by voluntary surrender unto Krishna for instruction — the subordinate bird immediately becomes free from all lamentations. Both the Mundaka Upanishad (3.1.2) and Svetasvatara Upanishad (4.7) confirm this