Nothing...especially if it effects the outcome of now. Just because you change something in the past ( let's say for the good of mankind) doesn't mean something else will go wrong. If anything I would go way back and try to maintain a healthy and an abundant supply of T-Rex. I have my reason >:)
Nothing. Whatever problem you MIGHT solve by changing the past, you will create others that may be far worse. Plus, it becomes a paradox. You go back in time to prevent Event A from happening. So, Event A never happens. So there's no reason to go back and do whatever you did. So you don't to back and do whatever you did. So Event A happens anyway.
I would change the past by helping Alicia,A girl in school that was alway's getting bullied, for mos.this went on before she committed suicide,and I have to live with the fact that I did nothing about it,So if I could change the past, I would of stepped in and helped her,and maybe she wouldn't of committed suicide.