"The effects of karma may be very old indeed. We are suffering or enjoying the results of our activities from time immemorial, but we can change the results of our karma, or our activity, and this change depends on the perfection of our knowledge."
"...The living entity forgets what to do. First of all he makes a determination to act in a certain way, and then he is entangled in the actions and reactions of his own karma. After giving up one type of body, he enters another type of body, as we put on and take off old clothes. As the soul thus migrates, he suffers the actions and reactions of his past activities. These activities can be changed when the living being is in the mode of goodness, in sanity, and understands what sort of activities he should adopt. If he does so, then all the actions and reactions of his past activities can be changed. Consequently, karma is not eternal. Therefore we stated that of the five items (isvara, jiva, prakrti, time and karma) four are eternal, whereas karma is not eternal."
Bhagavad-gita As It Is, Introduction
"Since the living entity is constitutionally part and parcel of the Supreme personality of Godhead, the Lord is very affectionate to the living entities. Unfortunately, when the living entity is bewildered or illusioned by the external energy, he becomes forgetful of his eternal relationship with the Lord, but as soon as he becomes aware of his constitutional position, he is liberated. The minute independence of the conditioned soul is exhibited by his marginal position. If he likes, he can forget the Supreme Personality of Godhead and come into the material existence with a false ego to lord it over material nature, but if he likes he can turn his face to the service of the Lord. The individual living entity is given that independence. His conditional life is ended and his life becomes successful as soon as he turns his face to the Lord, but by misusing his independence he enters into material existence. Yet the Lord is so kind that, as Supersoul, He always remains with the conditioned soul. The concern of the Lord is neither to enjoy nor to suffer from the material body. He remains with the jiva simply as sanction-giver and witness so that the living entity can receive the results of his activities, good or bad."
Srimad-Bhagavatam 3:26:16 Purport
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. Throughout his lifetime, the jivatma's aggregate desires are stored up in the subtle mind. These desires, along with karmic reaction, create the living entity's next body. Bhagavad-gita explains that just as in this lifetime the jiva soul transmigrates from child body to adult body to old body, similarly the jiva transmigrates from one species to another. Indeed, according to Vedic scriptures the soul transmigrates within 8,400,000 different species. And so long as the soul continues to identify with matter, it perpetually repeats this process of transmigration throughout all of these 8.4 million identifications.
"Other life forms are also subject to the cycle of birth and death, but when the living entity attains a human life, he gets a chance to get free from the chains of karma. Karma, akarma and vikarma are very clearly described in the Bhagavad-gita. Actions that are performed in terms of one's prescribed duties, as mentioned in the revealed scriptures, are called karma. Actions that free one from the cycle of birth and death are called akarma. And actions that are performed through the misuse of one's freedom and that direct one to the lower life forms are called vikarma. Of these three types of action, that which frees one from the bondage to karma is preferred by intelligent men. Ordinary men wish to perform good work in order to be recognized and achieve some higher status of life in this world or in heaven, but more advanced men want to be free altogether from the actions and reactions of work. Intelligent men well know that both good and bad work equally bind one to the material miseries. Consequently they seek that work which will free them from the reactions of both good and bad work."
Sri Isopanisad, Chapter 2
"The intricacies of action are very hard to understand. Therefore one should know properly what action is, what forbidden action is, and what inaction is. "One should understand the nature of karma, vikarma and akarma, and one must act accordingly. This is the law of the Supreme Personality of Godhead. The conditioned souls, who have come to this material world for sense gratification, are allowed to enjoy their senses under certain regulative principles. If they violate these regulations, they are judged and punished by Yamaraja. He brings them to the hellish planets and properly chastises them to bring them back to Krsna consciousness. By the influence of maya, however, the conditioned souls remain infatuated with the mode of ignorance. Thus in spite of repeated punishment by Yamaraja, they do not come to their senses, but continue to live within the material condition, committing sinful activities again and again."
Srimad-Bhagavatam 5:26:6 Purport
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.