When Martin Luther tried to change religion from within he soon found himself excommunicated. This seems to be the case for most people inside the Abrahamic religions who call for change. They are labeled a heretic and put outside the religion, if not killed.
Yet the prophets did/do exactly this (and yes, they were sometimes killed as well).
One person's heretic is another's prophet, no?
Is it possible to change a religion from within? Are some religions more accepting of this than others? How does a religion change if not from within?
In your own religion, what happens to people who call for radical change?
I would say that out of the organized religions (meaning having a social structure) none have been more prone to change than mine, Unitarian Universalism. Our theology, rooted in human experience, encourages change to remain always relevant to the times. Indeed, some have complained that we change too easily. (That is an interesting question too, how much change is too much, but not the subject of this thread.)
The reason why I bring this up is because even within UU, which is so prone to change, we have vilified our prophets, calling them heretics or worse.
Ralph Waldo Emerson had to leave his pulpit because his Unitarian congregation was not willing to follow his movement from viewing God as transcendent "Father" to viewing God as immanent "Spirit." But one generation later, the rest of American Unitarianism did catch up with him, and now we proudly hail him as one of our most cherished prophets.
We UUs proudly point to the abolitionists within Unitarianism, such as Theodore Parker and Moncure Conway, as evidence that we have a long history of being on the side of racial justice. We do (as do others). But we conveniently forget to point out that in their own time, prophets such as Parker and Conway were declared "radicals" and banned from the pulpits of mainstream Unitarianism.
We UUs currently are stuggling with the question of whether we are a church that abhors war but understands that sometimes in the face of grave injustice it's the lesser of two evils (just war) or we are more like the Quakers, a "peace church" that says that war is never the lesser evil (pacificsm). We can ask this question now but as the U.S. marched towards WWI, when John Haynes Holmes tried to preach his moral objection to all war, he was banned from the pulpits of a number of Unitarian churches as well.
The point I'm trying to make is that, yes, people can reform religions from within. It happens all the time, even with those religions that we generally think of as the most conservative. But as these people are in the process, as they are calling their denominations to follow, they very likely will be branded heretics or worse. They may even have to eventually leave the religion that they love. But *if* they are right, then their church will eventually follow, and it will have been because of them. Prophets cannot do what they do for glory and/or appreciation. They must do it because they feel compelled by their sense of justice on the one hand and their love for their church on the other.
In other words, fix yourself first, then you can fix others. 
