Curiously, the more I read about enneagram, the less it seems relevant. No please wait.
Enneagram relate to innate dispositions (unless I missed something). But most of the scientific data* don't seem to support the idea that it doesn't change during lifetime since we're better informed about epigenetics (and how the environment can affect gene expression) and brain plasticity.
Of course, our "spontaneous" reactions and the way we're dealing with external stimuli are partly rooted in our neural circuits repeatedly stimulated. And the brain still prefers following a path he already knows: then a simple habit (as an acquired tendency) becomes part of your personality.
Interestingly, the greek word "ethos" which originally meant a simple "habit", had come to mean a "character structure", as if the link between the two had been already understood by some philosophers. But I digress.
(Of course², some isolated alterations don't change the whole structure.)
However, the brain's also the theater of numerous genetic mutations - throughout our lives - allowing us to adapt to various environments (and perhaps causing mental disorders like schizophrenia).
So, if hardware's changing, then what about software ?