I think from a purely story point of view they won't let them change the past/future. Free will or not, variables or constants, changeable or set in stone, it doesn't matter.
From a story/series point of view being able to change the past in any meaningful way would negate the entire series. It would be just as bad as pulling an "it was all a dream" ending. And I give these show runners a bit more respect then to think they would do that.
So I don't think they will/can prevent the incident, or 815 from crashing, or Return of the Jedi from having Ewoks in it, or Charlotte from dying (or Eko, or Anna-Lucia, Charlie, etc...). To do that would be cheating the audience.