I'll also think about watching this one, to answer the question, this is a topic I've already thought about not so long ago. These are the facts:
You are who you are in this very moment thanks to (or because of) all the events which happened to you during all your life so far, which means that if you even slightly modified one of them, it could have made a really big difference in the long scale. It's the butterfly effect, a tiny change in your life can make a huge difference in the future, even decisive of your own fate and your personality, how it will evolve and how it will remain in the end, because it all adds up to what I call 'experience'.
Personally, I wouldn't want to undo anything I did in the past, I know I could be better, but I'm afraid that there are more chances of ruining myself by removing or adding certain events that if I do nothing. I wouldn't want to go to the past, not even to the future to see what I end up like, even if I keep saying "Knowledge is power", in this case it would be very likely to ruin your current personality. After all, we learn from mistakes, right? And if we undo the mistakes we did, sure, the bad thing didn't happen in the first place, but without you learning the lesson soon enough, you could be causing bigger trouble later on. That's my view on it.
Changing the past is changing your experience, think twice before wanting to mess up something as 'deadly' and decisive as that.
Great topic, by the way.