The funny part is that we tend to think of ourselves as this solid, whole thing that is a "being" and has a name and a fixed value of "I"ness, instead of a random collection of atoms doing different things, in a state of constant flux. The whole "I" idea is painted on, a psychological defense - or rather, a requirement for self-conscious beings to be able to function. So we are our own enemies when it comes to not being able to change, if you let go of your fantasies of self, anyone can change into almost anything. People stop changing when they think they've stopped changing, when their self-image becomes fixed in their minds, a mask that they stick to because it's familiar and comfortable.