Yes,
DCL will refund the taxes/fees. If your credit is enough to cover the fare+taxes+fees for the rebooked cruise, nothing more is owed (and any leftover credit will be OBC). If the credit is not enough to cover all of that, you will have a balance due for the difference. The credit is applied per-passenger - so if any passenger originally booked will
not be on the new cruise, a portion of the credit remains with that passenger for future use; if you add any new who wasn't on the original booking, the credit cannot be used for the new passenger's fare.
Clear as mud?