I am glad that your wife has recovered enough to sail.
I have had several experiences that may help. My father in law was in a very similar situation with his wife. He sent a letter along with Doctor notes to Carnival and they allowed him a full credit against a future date. This was several years ago and certainly is a case by case thing. He sailed later and the credit was honored just fine.
I also needed to change the names of travelers once. It cost $50 and was no trouble to do it.