Perhaps the main difference between a rebel and a reformer is that a rebel will usually attack and attempt to destroy the system they find objectionable while a reformer seeks to change it from within. The same person may attempt both tactics, and some of Emerson's acts and words could certainly be seen as rebellious. On July 15th, 1838, he told the graduating class at Harvard Divinity School that Jesus Christ was not God and that the miracles described in the Bible never occurred. This is certainly the act of a rebel, despite Emerson's relatively tactful language.
For the most part, however, Emerson should be seen as a reformer who sought to change people's minds through reasoned arguments and poetic eloquence. One of his lectures is titled "Man the Reformer" and points to
The duty that every man should assume his own vows, should call the institutions of society to account, and examine their fitness to him.
The emphasis on the individual here is always a feature of Emerson's work and suggests an important feature of his thinking about reform. Some of the public reforms Emerson advocated came to pass during his lifetime, the most significant being the abolition of slavery. However, Emerson was at least equally concerned with the individual's duty to reform himself. This was work that would never be finished, but, in the case of some people, such as his friend and protégé, Henry David Thoreau, this self-reform would lead inevitably to rebellion.