COMMENTS
We actually were offered a full refund on Frontier because the flight *number* changed. I couldn't believe it at first, but they called us and asked us to call back with our confirmation number about our flight. My husband called and they told him that the flight number (not flight time or anything else, just flight number) changed on one of our flights and because of that we were entitled to a full refund of the ticket. So my husband quickly checked online to see what the current price of the ticket was (I had been receiving notices that it had dropped by more than $100), and told them that we did want the refund. So they submitted the tickets for a refund and we turned around and bought them back for about $70 less apiece. When I received the notices that the ticket prices had dropped, I thought we were out of luck because the Frontier web site states that they do not offer refunds if the fare changes, so apparently this is a loophole in their policy.
Our upcoming Delta flight to Charleston SC was concelled. When I called they were apologetic but the replacement routing was very cumbersome. I did not accept their word and continued to press for a better routing. We wound with a more direct flight and a better time. Morale of the story: don't accept their first offer if it's no good. Be persistent but calm.