You can change, extend, modify any holiday you wish, just talk to a travel agent & they will arrange any thing you want. A holiday brochure is a book of ideas & suggestions, not the Ten Commandments.
Taken from Cunard brochure with reference to QM2 Transatlantic crossings. "Why not extend your holiday by spending as much or as little time as you wish in New York before or after your crossing?" This is followed by your options! Happy cruising.