Can you tell us the real reason you want to cancel?

On the face of it, your new itinerary is much better than the previous one. You even get a lie-in on the morning of departure!

Being in DFW for 6 hours doesn't really mean that you are entitled to be in Dallas for that long - it's not a stopover, so having this time compressed from 6 to 4 hours, when Dallas was never really a destination for you, is really a non-issue. I wouldn't argue that having an important meeting/event in Dallas on that day is a valid reason.

If you've just changed your mind and want to use the flight change as a convenient excuse to cancel, you are out of luck. If you've changed your mind and really don't want to fly, then it's a good thing you only paid €260 rather than the €1000+ it would otherwise have been!