CHICAGO (Reuters) - Flight attendants at Frontier Airlines are opposing an overhaul to the airline's operations - a key element of its turnaround strategy - saying the changes are not covered by their labor agreement and will "drastically upend" their lives.

In a letter to Frontier CEO Barry Biffle seen by Reuters, the Association of Flight Attendants (AFA) union said it is filing a notice of dispute under the Railway Labor Act that regulates labor relations in the airline industry. Such disputes are resolved between management and labor in a process overseen by the U.S. National Mediation Board.

Frontier has announced plans to rework its network to allow almost all of its planes to return to their stations every night. The so-called out-and-back model inspired by European budget carriers is meant to minimize flight cancellations and delays.

The ultra-low-cost airline estimates the changes will improve productivity and save $200 million this year, as it tries to return to sustainable profitability and deliver double-digit profit margins in 2025.

Frontier was not immediately available for comment.

The company's new operational model would be a shift from the typical method of U.S. airlines, which normally fly multi-day trips with two or three overnight stays for cabin crews.

AFA said the new plan amounts to a wholesale change in the airline's business model that would drastically affect the compensation and out-of-pocket costs of flight attendants, effectively resulting in a pay cut.

"The morale here has been very low," the union's head, Jennifer Sala, said in an interview. "The anxiety of the flight attendants has been very high in trying to figure this out for themselves."

Flight attendants make more money with multi-day trips, AFA said, adding that Frontier's plan could amount to a 20% pay cut for new hires. The union wants attendants compensated for financial losses if the airline continues with these changes.

"The changed business model includes changes not contemplated in the collective bargaining agreement and impacts all aspects of a flight attendant's work life," it said in the letter dated April 3.

Bringing planes home every evening will mean flight attendants will have to commute more frequently, boosting those costs, the union said.

Frontier pays $26 an hour to new hires, Sala said. Flight attendants are paid only when the aircraft is in motion. Their contract becomes eligible for amendments next month, but the union wants to resolve this dispute separately.