You don't change character. Character is already a nebulous entity, not cast in concrete: it's shifting and and responding all the time.
People may have some inborn capabilities and limitations; they have the internalized values of their earliest training; they have a temperament that they can learn to control to a greater or lesser degree. But nobody is purely one kind of person or another kind of person; we all have dozens of traits, proclivities, tastes, thought-processes, weaknesses and blind spots, virtues and strengths, some innate, some internalized, some adapted some cultivated deliberately.
Circumstances, from the first to the last moment of our lives, affect which of those traits are thwarted, which are stunted, which thrive, which are empowered, etc. This is why money, love, fame, alcohol, illness, hardship, power or humiliation affect different people differently - but they do affect everyone in some way.
And there are statistical indications that some traits which all humans have to various degrees are affected in a generally similar way by generally similar circumstances.