I had been able to change someone's opinion and that was my father in law. At first he didn't comment on it when I came out, then he said some things behind my back. He is not religious, he was really ignorant about it and had an initial bad opinion about it. My wife had some words with him about it and finally I did to. He thought I was turning her daughter into a lesbian. My wife has just come out to them as bisexual (she came out to herself in 2008.) I had a serious conversation about it and explained to him about being trans. I had known that he might have issue due to trauma his mother had given him. His mother wanted a daughter really bad and dresses him as a little girl till he was 7 (fighting with her that he wasn't a girl.) This was very helpful to me on explaining it, because for me it's like I was born a girl, but not only am I being dressed in the wrong gender, but parts of my body don't match. He slowly started to understand, but still questioned it but was at least pleasant to be around.
6 months later he and his wife came up to visit and saw me full time and doing well. We even went to the beach and my mother in law commented on how well I looked in a bathing suit. It was a really good trip and my mother in law (his wife) fully accepted it at this point, but he still had some issues. Unfortunately 5 months later, my mother in law was diagnosed with stage 4 cancer. (We found out 2 days before Christmas.) My wife and I were back and forth between NJ and TN (where they lived.) On my last trip back before my mother in law passed away, he took me to the side and thanked me for being there. He told me that he was shocked on how real this transgender thing was when they visited. He apologized for all the negative things he ever said about me transitioning and he ended it in saying: "A father can never have too many daughters."
Once he realized that we are human being and real people he accepted it. Unfortunately it took a tragedy for him to fully realize it. I think he would have eventually, but me being there during his time of need sped that up.