People ready to fly again may want to prioritize redeeming their airline miles, rather than trying to earn new ones.

A glut of unused reward miles may push airlines to change their frequent flyer programs in ways that could disadvantage some customers, warns ValuePenguin, one of LendingTree’s financial research websites. 

A review of annual filings of five U.S. airlines — Delta Air Lines, American Airlines, United Airlines, Southwest Airlines and JetBlue — shows reward program liabilities grew to a combined $27.5 billion last year, a spike of 11.6% over the year before, according to ValuePenguin.

Additionally, a rush to redeem miles is expected at a time when the badly battered airline industry needs cash-paying customers at the ticket counter.

Flying was one of the industries hit hardest by the coronavirus pandemic, but customers of the five analyzed programs still managed to amass about half (46.2%) the volume of miles in 2020 that they did in 2019. That was largely due to points earned from credit card spending, said Matt Schulz, LendingTree’s chief credit analyst.

Only a small portion of those miles were redeemed.

“Americans redeemed only about a tenth of their available miles last year,” Schulz said. “There were undoubtedly many people who … cashed in rewards points for statement credits for groceries and other necessities, but many people simply held on to their miles, waiting eagerly for the day when they could travel again.”

JetBlue customers redeemed the most miles (18.4%) last year, about half the miles its customers redeemed in 2019, according to ValuePenguin. United’s members redeemed the least, less than 9%, down from nearly 30% in both 2018 and 2019.  

With reward miles accumulating without being used, that 11.6% rise in liabilities translates to a total increase of $2.9 billion that took place last year, which is roughly three times the increase that took place the year prior, according to ValuePenguin.

Southwest’s Rapid Rewards liabilities grew by $1.1 billion, the most of any of the analyzed airlines. Its customers redeemed the most miles of all programs, but they earned the most miles too.

JetBlue’s TrueBlue program — the smallest of the five — was the only airline that saw its loyalty liabilities grow less in 2020 than they did in 2019.

Delta, United and Southwest have all devalued their rewards points during the pandemic, meaning members must now pay more points for the same flights, said Sophia Mendel, a travel specialist at ValuePenguin. And further changes may be in store.

“Southwest, in particular, is getting a significant amount of heat for having changed their points program without prior warning to its members,” she said, adding that several international carriers, such as Qatar Airways, have done the same.  

CNBC asked the airlines analyzed by ValuePenguin on Wednesday if they have plans to alter their rewards programs this year. Delta said it has no plans to change its loyalty program. Southwest verified to CNBC that since April 14 it has required more Rapid Rewards points to redeem flights on all types of fares.

United and JetBlue did not respond to CNBC’s inquiries.

Mendel said she believes devaluing miles or capping award redemptions is a response to airlines having too many unused miles on the books.

But Spencer Howard, founder of loyalty points website Straight to the Points, isn’t so sure. He said the risk of devaluation is always present, regardless of economic or global health conditions.

“Airlines devalued prior to and during the pandemic and will do so after we’ve gotten out of the pandemic,” he said. “Devaluations are inevitable.”

Howard agrees that customers have more miles now, but he said airlines can exercise control over when they’re redeemed.  

“If an airline thinks it will sell a seat for cash, it won’t release the award space,” he said. “As we come out of the pandemic, we’re already seeing airlines reduce the number of award seats available.”

But not every airline works that way, Mendel cautioned.

“Southwest and United don’t limit their award inventory, so any available seat is fair game to be booked with points or miles,” she said, noting that both airlines “use dynamic pricing, which means the cost of their award seats will increase as demand increases.”

While the “overwhelming trend” over the years is toward devaluation, Schulz said, not every airline is going that route — at least not yet.

Korean Air in January pushed its planned points devaluation from 2021 to 2023, in a move Mendel said demonstrated the airline’s commitment to its members over immediate revenue.

Last month, British Airways announced it was doubling the minimum number of reward seats on every flight. For example, the number of guaranteed rewards seats on Euro Traveler flights — the airline’s short-haul economy flights — will increase from four to at least eight seats. The new policy starts on July 28, though these flights can be booked now.

American Airlines, which was among the carriers examined by ValuePenguin, announced changes to its AAdvantage program last week that make it easier for members to earn benefits, said Andrea Koos, a company spokesperson. Members can receive 250 to 1,000 bonus “elite qualifying miles” on flights flown through Aug. 31, for up to 10 flight segments, she told CNBC via email.

Schulz said that flyers needn’t feel rushed to use their points, but they shouldn’t sit on a pile of unused miles for years either.

His advice: “Use points sooner rather than later.”

Miles and points can be redeemed for many different things, though most people redeem them for flights, said Schulz, adding “that’s typically the way to get the most bang for rewards points.”

Howard agrees that air travelers shouldn’t hoard miles, however he prefers another method to protect against changes to reward programs.

“I recommend people use credit cards that earn transferable points, such as Amex Membership Rewards,” he said. “This insulates you from devaluations as you have the option to transfer points to a variety of airline loyalty programs.”