Im not sure I understand the urge to disassociate. I hear usually two things. First, that the person wants "closure". Second that its the "principle".
1) Closure - Some people may get closure from this, but they have created ta mentality that will gain closure from the act. Embracing what you have been up to this point, and deciding to be something else, is fine. Nothing need be "closed". that is a misnomer. Don't cut your nose to spite their face. Just leave. Be who you are. Tell your family what you are comfortable with, and don't look back. No letter required.
2) Principle - What principle exactly? I used to eat peanut butter. Now I don't for some health reasons, and because I try to stay away from legumes. I'm an almond butter man. Have I written to the peanut council of america and explained why they have lost me? I realize that is extreme, but that is my point. there is no principle here. that is what JW's WANT you to think. I remember clearly being taught that if you were a catholic and you became a JW, that you better get your name "off their list". Why? That simply makes no sense.