Well, time -HAS- to have the ability to change, otherwise The Commission as an entity doesn't make sense. They want time to turn out a certain way, but that implies that time can turn out a different way. Plus, they have to have a reference point.
Although it could be a "journey is different, destination is the same". Kind of like the fixed points of time in Doctor Who. The details surrounding the fixed point could be different, but the end result was always the same. Like Waters of Mars where the woman was destined to die on Mars, but the Doctor brought her back to Earth...and she killed herself to maintain the timeline. It changed her whole legacy, but still has the same impact on her daughter in terms of her developing warp technology.