Shortly after not only travel through time, but also between worlds was discovered in the 26th Century, it was surmised that certain events could be changed without damage to the timeline. Instead just creating a parallel reality. With some experimentation, this was proven. Time Can Be Changed
I'm really confused as to why Elena doesn't know about the spell put on her and Bonnie. How come no one has told her? I feel like it's something really big and that she needs to know.