"The past" is not a fact, but an emotion: The past consists of emotional loaded pictures and stories. And YES, it is possible to change those emotions. You may say, it is possible to change the past.
At this point forgiveness occur, because there is nothing to forgive.
Forgiveness is always (also) Self-forgiveness.
Do I have to forgive others?
No, but I wished, they change their past, so that they do not need to forgive me themselves.