THEORY: How time can be changed according to the movie (SPOILER)
As long as the source code program is allowed to finish at the end of 8 minutes, nothing changes in the real world.
But if you kill the person who is being used to run the source code before the 8 minutes are up or right at the end of the 8 minutes, as in the movie, then real time is rewritten to whatever was happening in the last source code run.
At least, that is how I interpret what the movie is telling us.
Roll on the TV series. :)