No. Change is gradual. You may not see it right away, but nobody truly changes overnight. Even when we - as people - have those sudden, life-changing moments, they're the culmination of past events. Even when someone SEEMS to change in a moment, there had to be something building to that, even if they themselves were not aware. If someone seems to change overnight, the truth is you simply didn't see all the elements that factored into a more gradual change.

To be honest, the only person aware of everything that factors into a person changing is the person themselves. However, we rarely notice that we've changed because we're too close to ourselves. We experience every moment that leads us to change, so it's harder to see it. But anyone outside of ourselves can't see what's going on in every aspect of our lives. They aren't with us every second of every day and they don't hear our thoughts. They don't know all of our fears and hopes, our insecurities and secrets.

So, again: no. Barring medical issues - physical or mental - that I'm not knowledgeable to comment on, I do not believe it's possible for anyone to change overnight. It can seem that way - to both people around a person and to the person themselves - but an examination of the events leading up to the apparent overnight change will show you that the change was coming for some time.