You are in a gray area. DoT Regulations require them to refund to your original form of payment if the flight is cancelled (a flight number change would indicate cancellation of the original flight and the initiation of a new flight) or a significant change in time. Whether 2 hours is significant is ambiguous. I think for example 2 hours on a cross country flight might not qualify but a 2 hour change on a short flight might. I don't know if it will help but I'd contact them in writing listing the multiple changes in time and if applicable cancelled flight number(s) (maybe via social media like twitter), remind them of the DoT rules and specifically request a refund in your original form of payment. If they decline you can try a Credit Card Dispute and file a DoT complaint. Fortunately you have some time before the flight.

Good Luck and let us know what happens.