Honestly, I never am "certain"... I tend to constantly mull over the events and plotlines in my head, until it is time to "put them to paper", after which, as a rule, I stick to what was written.
I have already revised some of the earlier events slightly, and I constantly looking back at the old events to see if I can tweak a thing or two, maybe add another sentence or put in a choice. But I don't think there will ever be any major re-writes. It is really difficult for me to change what has already been written. Not in the emotional, but in actually "writing things" way. But there is still a lot of ways I could improve those earlier events, I just need to find ways to do so.