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.
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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.