The US is a young country, so comparing the amount of changes to the Magna Carta to the Constitution is not as relevant. Especially since the United States was molded after tried methods experimented in Europe. Plus the United states has had some massive changes in its constitution since its creation.
Segregation, Slavery, The right to sign Contracts, Search and Seizure, Myranda, Speech, Religion, Gather, Press, Bear arms, workers rights, the right to protest, Minimum wage, Double Jeopardy, The worth of Black men and women, Woman's rights, Gay and transgenders rights, The right to a trial, Innocent until proven guilty, No cruel or unusual punishment, The right to vote,Child Labor laws.
These were all major changes in our history, especially after the articles of confederation fell through.