First-class upgrades, free booze and access to airport lounges may be harder for some passengers to earn if they don’t begin buying more expensive airline tickets.

United Continental Holdings Inc., the world’s largest airline, today joined Delta Air Lines Inc. in setting spending criteria for its MileagePlus frequent flier program’s elite levels, which grant passengers extra perks such as upgrades and shorter check-in lines.

Starting in January, United passengers must spend $10,000 and travel 100,000 miles or 120 segments in order to gain the top status of Premier 1K, Chicago-based United said in an e-mail to customers. Previously, passengers could attain that status by hunting down the cheapest tickets or by taking a few long-distance flights that racked up miles faster.

“There are going to be winners and losers here,” Tim Winship, the publisher of FrequentFlier.com, said today in an interview. “The losers certainly are those people that have traditionally earned elite status by making mileage runs, long cheap flights that people take solely for earning elite status or elevating from a lower tier to a higher tier. And indeed these changes are designed precisely to winnow out those folks from the ranks of United’s elite ranks.”

Delta, the world’s second-largest airline, made a similar switch to its loyalty program in January. United’s move could see all major airlines eventually migrate in that direction, said Jay Sorensen, who runs IdeaWorks, a Shorewood, Wisconsin-based airline consulting firm.

“At some point we’re going to see miles go the way of the buggy whip and they will no longer be part of frequent flier programs,” Sorensen said. “This is going to upset people who are miles junkies tremendously, because they really have tapped into this awkward method of measuring the value of a customer.”