Can individual humans change? Or are they stuck being who they are forever? Do humans have a personality equilibrium? Is it that they can make short-term changes only and then gravitate back to their original personalities?

It is pretty clear that humans can at least make short-term changes in themselves. The question is whether they can make long-term changes which in effect make them into different people.

Do human personalities travel a steady line in which changes in personality are only short interruptions - always tending back towards a base personality? Or do human personalities travel in a branch-like way in which they travel down different branches away from the original personality?

It seems that long-term changes in individual humans are difficult. Once a change is made there is a tendency to relapse to the previous behavior. And in order to keep a change for the long term it looks like constant maintenance is required in order to keep that change.

If the change brought about positive conditions, the maintenance might be very low to none though. This is because the resulting positive enforcement maintains the change itself. If on the other hand, you don't see much benefit from the change, it may be difficult to impossible to maintain.

Another question is whether the human really changed if rigorous maintenance is required to keep the change. If given the chance, he/she would go right back to their previous behavior - so is that change?

In summation, it at least looks like humans can change if they see the change as positive and it brings about positive reinforcement.