I think all of the time travel in the movie is awesome and all, but no, the past is never changed. Any time travel into the past in the context of the film is a predestination paradox; always happened and always will.
...unless you care to think that Jones may have stumbled onto something new, but we don't get to see where things go with her, or from the "present" perspective where things might have gone with her.
Might have been interesting if Cole had run into Jones or another time traveler in the past before they had been sent into the past, so he could tell Future Folks that they would be sending someone into the past.
You know, as fond as I am of the theory that time travel into the past means that you've crossed over to a parallel timeline, I think in this particular case I'm not going to spend any time trying to unravel this movie in such a manner.
Also, I seem to remember the movie credits mentioning that there was a novelization. Does anyone else recall this, much less seeing the novelization itself? Because that would be...well, I don't even know...