All you have to do to change cabins is to call
DCL and request it. If you are just changing within the same category, as long as a cabin is available, you can do it. The only thing I can add here is sometimes if someone is merely looking at a cabin on the DCL site, it may not show up as available for 20 minutes or so. You can go in and see what is available by plugging in the date you are cruising, category, and number of people. Sometimes more is available as the site tends to only show up to 8 cabins, even if more are open.
I'll be cruising concierge in February and April of 2012. And then most likely in January of 2013. I'll work on doing a trip report for them. (I'm not famous for finishing them....
)