I just rebooked for the first time. I have always booked new cruises within the 90 days and so never had the "fun" of rebooking to get a lower price. I just booked a new cruise, choosing the cabin next door and saved about $400. And then called and cancelled previous booking.
One thing I noticed is that adding the Plus package at booking changes the deposit. Without the package the deposit was $99, but if adding the Plus package the deposit went up to $400. I guess there is no way around having to front Princess the cash by adding a package.
All in all, about 30 minutes of my time to cancel and rebook, but I saved $400...