I know from personal expirience that people DO "just change" at the drop of a hat.
It depends on situations and expiriences and things that happen.
I have recently took a change for the worse I think, because I've been thrown into a new situation that I'm not used to.
I'm not the same girl I used to be.
Or maybe this isn't a permenant change, maybe it's just a temporary funk.
But I know that I've changed.
It REALLY all depends on context and situations. People change as they grow, and their lives change, thier personalities will change.
Sometimes it's more of a gradual change, but I even notice in myself a change that happened very suddenly.
So my opinion is that, yes, people DO just change.
A cheater, can reform their ways depending on the person they are dealing with. If they truly love someone, they WILL change. As they grow older they will change and as their lives and situations change, they will change.