As much as there is to hate about flying, one remaining pleasure of air travel is watching your frequent flier miles accumulate, bringing you that much closer to a cherished free trip—or so you thought.
Reminding travelers everywhere that awards charts are fleeting, United Airlines and Delta Air Lines this month raised the number of miles needed to score certain flights, essentially devaluing all those carefully earned points in their customers' accounts.
The changes have many fliers wondering which carrier will be the next to "update" its loyalty program, with some eyeing the merger between American Airlines and US Airways as a possible catalyst for more such announcements.
(Read more: US Air, American deal aids competition)
Airlines generally change their programs once every two to three years, said Tim Winship, publisher of FrequentFlier.com.
"Whatever your miles are worth today, they are almost certainly going to be worth less in the future."
"Whatever your miles are worth today, they are almost certainly going to be worth less in the future," Winship said.
"Travelers have to take seriously the verbiage that is included in the terms and conditions of every airline loyalty program out there to the effect that 'we the airline reserve the right to change this program at any time for any reason.' That isn't just bluster."
Devalued miles don't mean you'll have a harder time finding an available award seat, just that you'll have to give up a bigger chunk of your account to book it, Winship said.
He called some of United's changes "eye popping," but noted they will have little impact on coach fliers. Instead, the new rules—which apply to any MileagePlus awards tickets issued on or after Feb. 1, 2014—will "astronomically" boost the prices of some business and first-class tickets, said Gary Leff, a loyalty program expert who runs the blog, View from the Wing.
"There's very little change to awards in economy. It really focuses on premium cabins," Leff said.
You'll especially feel the pain if you want to use United miles to book a ticket on United's Star Alliance partners. Want to fly roundtrip in business class from the U.S. to Europe on Lufthansa? It'll cost you 100,000 MileagePlus points now, but come February, you'll need 140,000 miles for the same trip, a 40 percent increase.
Delta's SkyMiles changes are less dramatic, but they still shocked observers since the airline already announced tweaks to its program in August. Those changes boosted the amount of miles needed for some business-class tickets and will take effect in June.
The latest update, which affects award seats booked for travel between February and May, requires travelers to hand over more miles if they're flying to Hawaii using Delta's "Saver" awards, plus other changes.
"(Delta was) too anxious, they couldn't wait for higher prices," Leff said. "They led everyone to believe that you were good with the old rates for travel through May and they changed their minds on that. So that was hugely surprising."
Leff—who refers to SkyMiles as "SkyPesos" because he believes they're worth much less than competitor currencies—noted the big problem with Delta isn't its award pricing, but the amount of seats it makes available in exchange for miles. In a recent survey of award seat availability, Delta tied for last place with US Airways among 25 airlines ranked by IdeaWorks.
(Read more: How you're being nickel and dimed by airlines)
With US Airways on track to merge with American Airlines, many members of those carriers' programs are wondering whether their miles will be devalued next. For now, each airline will maintain its current loyalty program, American says on its website.
Leff believes the combined carrier won't take the "customer-unfriendly steps" that United and Delta have taken with their awards charts. But Winship warned that airlines tend to move in lockstep with such changes.
"The very fact that the two other largest U.S. carriers have announced price increases for their frequent flier programs basically opens the door for the new American to do the same," Winship said.
"The takeaway from that is: Redeem (miles) now rather than later … as soon as you reach an award level in your account, cash the miles out. Don't get into the hoarding mentality that so many people in these programs have succumbed to."
(Read more: Connected from the clouds: 3G and 4G on planes)
—By NBC News contributor A. Pawlowski.