That would only happen if we're thinking in "one universe" theory, in which case yes it will replace its own future (becoming a guaranteed truth when coming back to present time).
But if we're thinking in "infinite alternate universe", when they changed something in the future and then come back to present, they are on an entirely different, new timeline, which will go to this new future they created by making changes in the original timeline's future.
Ex: They depart from present time of point A to point B in the future. When they make changes on this point B, it becomes a new future, point B1 (it does not overwrite point B). When they came back to the present time, they don't come back to point A, they come back to point A1, which created because they are now in another new timeline (from making changes), in which will go towards B1 timeline.