As I understand the body completely reproduces its cells every so often so that we are in fact completely new creations at some point on a continuous basis except for the brain. The aging gene never goes away but we do have new organs and so forth in one sense they are just older each time they recreate.
Since I took up cycling many changes have occurred in my body. The idea that I have a new heart seemed like a good analogy. I was getting to a point where I could be classified as having borderline high blood pressure. I did not want to take pills. So much so that I started riding my bike 100 miles a week, then 150 then 200. Now my heart beats slower, my blood pressure is lower and in theory I should live longer. I can ride longer and faster and it still beats slower than when I started. Even though it is older it is better than before. Even so I could drop dead at any time from some unseen condition or be killed by a careless motorist or accidental fall. Training our minds and spirits is much the same except we are the ones in control. The things we do or don't do have an effect on us.
We refer to the term "a mighty change of heart" as spoken of in the Book of Mormon. This is a metaphor that refers to a time when we begin to see and feel beyond what the natural man sees and feels. It is not a physical event. It is occurs when we take the step to reach out into the infinite, when we let our mind think of eternal truth and ponder. It occurs when we seek God in study and prayer to understand things we would not otherwise be capable of understanding. I have felt it and seen it in myself and my children. It is a most amazing event. It too requires continued attention to be maintained.