The answer depends on when you are making the change -- is it before the Paid In Full date (when penalties start) or is it within a couple of weeks of sailing when penalties are pretty steep?
It also depends on what change you really want to make. Do you only have 2 people in a cabin and want to change one of them? Or do you have 3 or 4 in a cabin and want to cancel one. Were you thinking of bringing someone else just to fill the space because you thought it would be cheaper to change names than to have to pay for that spot anyway?
Generally, if you are looking to cancel someone and it would leave you with only one person in the cabin, it would be better to do a name change. (If you cancel and then add someone, you do it at the current rate, not at the rate when you first booked the person that is cancelling. So using the "cancel" word may cost you more even though in the end the same number of people are sailing.)
Not sure if this makes any sense but to give you better advice, would need to know more about the circumstances you think you will be in to cancel or replace someone.