Constitutions are changed often based on a majority vote.
Not with a simple majority of course, usually it's much harder to change a constitution.
(or, again, it falls back on the "it's not really a constitution" clause, as hinted before)
Uhm, sure you need 75% here I think. So 75% will steal from 25%
much better.
Democracy with a majority model is just a completely flawed model.