The way you naturally naturally react to and see things in a conceptual sense, which is basically your temperament, probably can't change unless you go through extreme measures like taking psychedelics or undergoing heavy brain damage that manages to affect just the right spots of your brain, which even then I'm wouldn't be totally sure if it will actually change your natural temperament. The thing is your natural personality doesn't truly decide your surface attributes which I would say that the Big 5 personality system partially covers. So with enough effort and favorable factors, you can definitely change your surface personality like becoming more open, agreeable, and less neurotic. I personality tend to fluctuate in my traits of agreeableness, openness, and extroversion.
I can't fully confirm this, but I feel like I know a lot of people who have many trained behaviors that probably didn't feel natural to them when they were younger, and I don't mean the basic stuff like having tact or displaying a basic level of politeness and respect, who of course are all above the age of 25.