You will definitely be under penalty to cancel at this point. When your cruise is scheduled depends on how heavy the hit will be.
As a T/A my first suggestion when I encounter a client with this scenario is to see if there is someone else who wants to go in the place of the person who is not going to go. Carnival will charge $50 to make the name change. That would minimize the financial impact. Insurance is typically not transferable so you would probably have to buy new insurance for the new guest.
You can not switch both passengers in a double cabin, only one.
Here are Carnival's cancel penalties:
2-5 Day Cruises
Up to 61 Days Prior ~ No penalty (Except Early Saver)
60-46 Days Prior ~ Deposit amount
45-30 Days Prior ~ 50%
29-15 Days Prior ~ 75%
14 Days or Less ~ 100%
6, 7 & 8 Day Cruises
Up to 76 Days Prior ~ No penalty (Except Early Saver)
75 to 46 Days Prior ~ Deposit amount
45 to 30 Days Prior ~ 50%
29 to 15 Days Prior ~ 75%
14 days or Less ~ 100%
Alaska Cruises, Cruise Tours and 10 Day And Longer Cruises
Up to 76 Days Prior ~ No penalty (Except Early Saver)
75 to 46 Days Prior ~ Deposit Amount
45 to 30 Days Prior ~ 50 %
29 to 15 Days Prior ~ 75%
14 Days or Less ~100%
Penalties for 14-18 Day South America and 14 Day Panama Canal Will not change.
