How much can a person change his sexual orientation?

Sure. You learn a new hobby, and your brain adapts accordingly. But what we're talking about are two different things. While playing the violin may make some parts of the brain work more, and therefore evolve over time, someone's sexual orientation is defined by their overall brain structure, not certain elements & areas, as well as hormones throughout the person's life. Overall brain structure is linked to hormone levels during prenatal development.