I believe that you will note that Cunard along with all shipping lines rely on well established conventions that allow them to change an itinerary. I believe it's the Athens convention though being away from my main office I would have to check.
The question is when did Cunard know they were changing to Newport and how did they communicate this to passengers. Convention dictates that they should notify you within a "reasonable" period. If Cunard themselves didn't know until 24 hrs before then clearly it wasn't possible for them to notify 2600 passengers in advance. If on the other hand they had say 2 weeks notice and the change could be considered to be a significant change affecting the overall basis of the holiday then i would think there is a reasonable case.
Having said all that I believe Cunard made reasonable efforts to ensure you had a most enjoyable experience. A considerable sum of money was spent of fireworks and the main facility of the ship was still available. I would think that "security" will be used a s the reason for the change. Sadly changes post 9/11 are impacting on many travel plans and Cunard must change thier plans according to advice and most possibly in this case because they had no alternative.