Allegiant allows passengers to change their flight itinerary for a fee. The cost to change a flight can vary based on the original ticket price and the new flight's cost. Passengers can make changes online through the Allegiant Airlines Change Flight or by calling the airline's customer service. However, it's important to note that some fare types may not be eligible for changes or may have restrictions. It's always best to check the specific fare rules and regulations before booking or changing a flight with Allegiant.