You can rebook. It will be a new reservation, but they will move the money over for you. Most of my cruises have been restricted rates including the last one on August 30. I would do it in a heartbeat to save 400 dollars. People make this big deal about not being able to select your room, but I've always ended up in better rooms than what I would have booked. It's usually the most expensive rooms that don't sell out. We got placed in a 5b midship deck 8 on our last cruise. I don't know what category you are booked in to begin with so that may make a difference.
The room will be nonrefundable when you book it, but insurance will still apply. At some point all rooms become non-refundable. I booked my last cruise at two weeks out and it wouldn't have mattered if I booked a VGT or any other room they were all non- refundable. Like I said insurance will still cover you if you go with a VGT.