I agree with 1153rsmith about finding out if it's worth it to cancel what you have on hold and move your dummy date to it. That's the whole point of booking a dummy date. Also the pp you quoted has 3 cabins booked so that's $200/cabin, which is what
DCL will give you when sailing a cruise that was booked onboard (even if you change it to a different sailing later on land, it's still the same res # therefore OBB perks follow it.)
There are TAs that will give you OBC just as a thank you for booking with them which can be stacked with the OBC from DCL.
We'll be platinum by then so we'll get $350 from DCL + $200 from TA = $550 for 1 cabin.
If you didn't request a TA at the time you booked the dummy date, you can probably make the change online yourself. If you really like the cabin you have on hold, just go into that reservation and change to a different cabin which will "free up" the one you like. Then modify your dummy date to the desired date & recapture the cabin you like.
Good luck!