We had a tour operator pull a pretty similar stunt on us, although they left it a whole lot later in the day to inform us.
We complained; they ignored our letters. We sued; they found it to be an expensive exercise. At the end of the day, it cost them a whole lot more than it would have done if they had acted fairly and offered a reasonable amount in compensation at the beginning, rather than playing the "deny all claims" card & trying to wriggle out of their responsibilities.
Given that they have told you about the changes well in advance, what might be worth considering perhaps at this stage in the game is to threaten to take your business elsewhere unless they are prepared to offer you reasonable compensation for the nuisance and inconvenience caused by the repeated changes to the flight times.
I would also suggest that you pay a visit to their website to see if they are selling the holiday any cheaper now that they have shifted the flight times about, as if they have, this gives you leverage for making a case for compensation.
Whatever else may be at issue, you are certainly entitled to have your money back in full under the circumstances if you would prefer to shop elsewhere for a holiday with more suitable flights and dates, although you do have to act sooner rather than later if you want to exercise this option.
If you do write another letter of complaint, put "without prejudice" at the top of the letter. This gives you the right to ask for additional compensation if they mess you about with more changes at a later date.