Well, I got it done...here's what I did:
First, I booked a brand new reservation for the same sailing date in the same cabin catagory (10A)...this was hit and miss as it disappeard from being available for a while this morning, but then showed back up and I was able to book it...
DCL gave me a new reservation number.
Second, I called up DCL and told them what I wanted to do. They had no problem transferring my secret porthole cabin to my new reservation number, along with the deposit I had already paid, then cancelled my old reservation. As mentioned by canadian_mouse, it isn't an onboard credit, it just means my balance owing is lower.
The only downside was they weren't able to transfer my early dining reservation...the moment we cancelled our old reservation, our early dining slot went to the first family on the waitlist....however, we were added to the waitlist, and I'm hopeful that with almost six months to go that enough people will cancel and that we'll move up the waitlist and get back our early dining.
The best news, of course, is the savings! For our family of four, it's US$675!