I simply took my old box to the office, and they took it back, gave me the new one, updated my account in the computer, I signed the reciept saying I returned box with S/N: xxxxxxxxx and recieved box with S/N: xxxxxxxxx. Very easy. You might want to ask if they have any of the newer DVRs in stock. Tell them the new ones are black, they will understand that(if you start asking for a DCX3400m or one with the 500GB drive odds are they won't understand). I tried to activate box on digitalnow.comcast.com (The new DVR showed up on the list of devices) but got impatient after 10 minutes and just called, and they took care of it. It was sooo obvious the gal was reading a script it was painful to listen to her.
You could go get another one and bring the older one back in a couple days(say, to finish watching your shows), and you would just have a day or two's charges of the 2nd DVR(which would only amount to $1 or so, but I bet they screw something up). It's easier for me to make 1 trip to the office and be done.
The DCX boxes do have a new remote(platnium/dark gray). As I understand, those new remotes do not work with the older DCT boxes, but the old remotes/IR codes do work with the newer DCX boxes. So in your case you should not have to reprogram your Harmony remote(s), but don't quote me on that.