I don't take this list to be especially significant. It's one of the more annoying points to make in an argument that while I understand where you're coming from - I used to believe the same thing! - I now see where I went wrong. I am not impressed that C. S. Lewis used to be an atheist. But here's a list, roughly in order of personal significance.
- Stopped believing in God (I was very religious).
- Became a "vegetarian" (some fish and other seafood) and general animal rights nut.
- Went from regular lefty to dogmatic libertarian (this meant changing my mind on many other things as part of the deal; most eye openingly, for no particular reason, about the minimum wage. This also leads to no. 4.)
- Used to be very anti-gun. Now I think gun laws are too restrictive.
- Inexplicably used to be against getting a cell phone.
- Hugely revised my worshipful attitude towards modern medicine in general and doctors in particular.
- Admitted to myself that I generally hate the type of movies they show at Cinema Nouveau and most plays.
- Changed from being Liverpool to Arsenal supporter (situation is still fluid though).
That'll do for now.
Added: Not sure how I forgot this, but very recently I changed my mind about free will. I now think we have it. This is difficult to rank though, because I've spent ages thinking about the question, but I've never been emotionally invested in either position.