Good question OP. I was wondering about this for our Alaska cruise in July. I booked the White Pass train excursion in Skagway for all 16 of us but wondered what would happen if the price dropped and I cancelled and tried to rebook. The price is pretty reasonable through RCCL, and not much different than third party vendors, so I'm not sure it's worth the risk.
However, for things like soda packages or drink packages, or dining packages that don't sell out, there is virtually no risk to cancel and repurchase. I've done that many times.
Dan