To reiterate what others have said, if the airline changes the schedule of a ticketed itinerary, they are obligated to allow you make changes (other than the ones that they made for you) in order to meet with your requirements. Certainly, either departing no earlier or arriving no later than the original flighs is a reasonable accomodation, depending on which side your time constraint exists. Actually, their only obligation under the tariff is likely to be a refund of your money without penalty, even on a non-refundable ticket. In practice, they are more than willing to work with passengers to adjust the schedule. Just make it clear to the agent that the requested change is due to a schedule change on their part.