Costa Firenze was transferred from Costa Cruises to Carnival Cruise Line in Cádiz, Spain, on February 2, 2024 and underwent a 49-day dry dock at Navantia Cádiz that transformed the vessel into Carnival Firenze. The conversion integrated Carnival's Fun Italian Style product while retaining the ship's Florence-inspired Italian design, with Carnival branding, dining, beverage, entertainment and guest-experience concepts incorporated throughout the vessel.

Notable additions included the Il Viaggio specialty restaurant and Festa Italiana programming. The conversion also introduced new voyage-efficiency technology, making Carnival Firenze the first Carnival ship to use an artificial-intelligence-assisted system that provides officers with recommendations for more energy-efficient routing. After completing the Cádiz dry dock, Carnival Firenze departed for California on March 22 and entered passenger service from Long Beach on April 25, 2024.