Question: “Does the gandhabba undergo a constant change from the time of conception till the death of the body
or does the change happen only at the time of death?”
Gandhabba can undergo change both while inside a physical body or while outside (while waiting for a womb).
– Drastic changes in gandhabba can happen while inside a physical body (like us) IF we attain stages of Nibbana. If one attains the Sotapanna stage, then that gandhabba will never be born an animal or one of the lowest realms (apayas). Of course, the most drastic change would be to become an Arahant.
Question: “You mentioned karmic energy, is the energy cumulatively formed for the next body, to take it to the next destination (animal, human, deva, and other forms)?”
For humans, kammic energy is acquired at any time, especially while in a physical body. Each of us has done many, many good and bad kamma in our past lives. All those can potentially bring in kammma vipaka.
– Of course, the strongest ones have the priority to bring vipaka.
– For example, if someone kills one of the parents, that kamma will lead to a birth in an apaya when that person dies. It will not wait to bring vipaka in a future life, because it is one of the strongest bad kammas (anantariya kamma).