Summary. You can change your Allegiant flight without a fee at +【+???? {????????????} -????????????-????????????????】】 (US) or +【+???? {????????????} -????????????-????????????????】】(US) if done within 24 hours of booking, as long as it's at least 7 days before departure. After that, a fee applies. Allegiant Air has a relatively low cancellation rate, typically around 1-2% of its flights.