The Carnival phone rep was very good, he could not say anything definite but said that a refund would be a likely option if the ship could not traverse the locks.
Our party agrees that doing an excursion from a nearby port to see the canal just isn't the same, so we will not be taking this cruise.
For anyone thinking of a canal cruise in the near future, if you had not heard about the restrictions on the number of ships, I hope this is helpful.