I realize the original question was posted in 2008, but it's never too late to reply, right? There seems to be general consensus that, scriptural/symbolically speaking, heart refers to the "inner person". Though the "mind" is broader than the "heart", the heart is the core of the inner person and is a place within each of our minds where we decide to cultivate and maintain our true thoughts and beliefs about things. The heart is a place in our thinking that is genuine (for better or worse); a place that we do have control over; it is the place where the Spirit delivers inspiration to (only "to" because we have to be willing to "let him in"). Out of the heart flows desires. Our behaviors, words and expressions might "suggest" to others that we have a particular belief, but our heart is where our true thoughts exist. Hence, a "mighty change of heart" is virtually synonymous with the Greek word "metanoeo" (the word for repentance), which is most often translated to mean "to change one's mind. . .for the better". A mighty change of heart isn't something that simply "happens" to us without our permission. This mighty change in heart happens when we change our mind about the things of the world AND the things of God and determine to follow Him in faith. Then, what makes this change "mighty" is the Spirit and, subsequently, our views of God, doctrines, principles, etc, as evidenced by Swelling or Enlarged or Enlightenment or Delicious (S.E.E.D. as found in Alma 32). My 2 cents. . .
