I think you are missing the fact that in its almost 250-year existence the United States has changed significantly many times. Only one of those required widespread violence. The rest were all accomplished through mostly peaceful lawful means. The changes were not always easy, for example, the civil rights movement was very challenging for the country but was fundamentally peaceful.

As one example of how much the US has changed, you can review the constitutional amendments that prove the point. The 13th, 14th, 15th, 17th, 18th, 19th, 20th, 21st, 22nd, 23rd, 24th, 25th, and 26th amendments all represent significant changes in culture and democratic structure.