You can change your Frontier Airlines flight date by contacting their customer service at US. Provide your booking details and the new date you prefer.....
https://www.wetravel.com/trips/1-866-863-7444-us-how-to-date-change-on-frontier-airlines-flights-mario-smith-41968151

ALLEGIANT Airlines’ Group Reservation Number flexible booking options and personalized service aim to streamline the travel experience, providing ease and convenience for those embarking on....
https://forums.besttechie.com/topic/47633-allegiant-airlines-group-reservation-number-how-it-works/