Any contract that allows one party to change it at will is not going to hold up in the court.
DL, and any airline, would not dare to have their CoC be examined line by line in an open court.
You simply don't understand. DL isn't changing it's contract w. OP. DL is changing the flight time and that is expressly permitted under the contract.
The world's full of people who give legal opinions without having done the research. If you've got some research to share, that would be great. Otherwise, why not provide OP some constructive help.