[I]s it possible to change the joists, supporting walls and foundations or who we are? What we believe, value and aspire to in life?

Yeah, and it's also possible for them to rot and collapse. The question begs for an affirmation with the end result being positive. My recent ex, who will no doubt read this at some point (Hi, honey!), seems to have responded to traumatic life events by becoming a hateful, bitter and violent human who bristles at the slightest criticism. Bristling can include a softball sized bruise delivered to the sternum of a nearby human BTW. And given the opportunity to heed red flags and advice from historical experience of basically every human ever coming out of the experience she was trying to free herself from, chose to return to the comfort and familiarity of misery rather than the scary world of change and independence, essentially dooming herself to becoming more bitter and more miserable.

My dad is a shit human being. He was maybe less shitty when he was younger. He volunteered to coach youth football when I was a kid. I don't know that he hit my mom until his late thirties. Over the course of his life his experience didn't square with his expectations and he's a crazy, broken old man now who I haven't spoken to in almost two years and the last time was pretty much serendipity so aside from one encounter it's more like 4-5.

In my experience, the capacity to change for the better is hinged on an ability to be self-critical that not all people are willing to go through. And even if a person is aware of their shortcomings the awareness itself can be cripplingly depressing. It's easier to just ignore the difficulties in life than to address them.

"When you stare into the abyss, the abyss stares back," isn't an instruction not to examine the dark parts of the world and your life. It's a statement that honest self-appraisal is fucking frightening.

Yes, most people change. No, your husband who throws you against walls is probably not going to do a 180 and become an awesome dude. The second sentence there is the honest answer people googling "Do people change?" are not truly looking for. What they're really looking for is justification for remaining in the company of toxic people. False hope is awful. It can be a punishment you give to yourself.