To what extent are you planning on "changing your personality?"
I think it's pretty odd, possibly psychologically damaging, to all the sudden say "I'm going to force myself to look different, act different, do different things, and believe different things starting tomorrow and I'll never be any of what I am today."
I imagine you'd have a number of "who am I" moments with bouts of manic and/or bipolar-like episodes.
If you're saying something like "Tomorrow I'm going to stop being the sarcastic asshole who is too passive for his own good and start being the sincere, assertive but gentle person" I think you'll do just fine.
I could imagine trying to force myself into a few different behaviors, but I'd never be able to completely change from who I am now to someone else.
Why do you want to do this? What are you hoping to accomplish? What's the end goal?
I'm not going to change how I act or look at all though. I wouldn't be going so far as to change who I am as a person, but more to change parts of my personality.
I don't think you can really change your personality, per se, but you can change how you act. For example, I have really terrible social anxiety, but I made a conscious decision five years ago that I was going to stop letting it rule my life and have tried to force myself into extroversion. I didn't change that part of my personality, I regulated it. (for the most part-- it's an uphill battle.)
But you're going to have to be more specific. What are you trying to change? (Maybe "modify" is a better word here.)
When I was in middle school, I did what you did, and I just forced myself to be a more outgoing person, and be loud and out there, and I did, and everyone always tells me how different I am now than I was when I was younger.
Basically, I've always been Larry David-like, in the sense that I'm pretty relatively unliked, even though I am liked by people I know? I realized that personality doesn't really work, as I really haven't met a new friend or person since I graduated high school, just because I come off like such an asshole. So I don't want to be "nicer" but I do want to be more likable, whatever that means...