Delta's frequent-flyer program on the descent

FOR BUSINESS FLYERS like Adam Kubryk, who is on the road about 200 days a year, one of the perks of flying often and using the same airline regularly is getting upgraded from coach to business class when a seat is available.

But now when he flies on Delta Air Lines on some of his more frequent work-related routes, he has lost that key perk, and it is making him mad enough to ditch the airline and take his frequent-flyer miles elsewhere.

The issue: In March, the airline stopped offering upgrades for flights between New York and Los Angeles, San Francisco or Seattle.

Delta had previously offered upgrades on these cross-country routes to high-level frequent flyers when the seats were not sold. Now frequent flyers say they are seeing empty business-class seats while being told that they are no longer available to passengers who do not specifically pay for those higher-priced seats.

"It blows my mind that they'd rather keep those seats empty than keep my loyalty," says Kubryk, 31, a New York-based hotel furniture salesman.

Undoing upgrades on those routes comes amid a swirl of other changes for frequent flyers, but particularly those who fly Delta. The biggest change, calculating points earned based on money spent rather than miles flown, favors business travelers who are the most likely to pay top-dollar prices for last-minute bookings.

The change in how miles will be calculated is scheduled to start at the beginning of 2015. The airline also revised its frequent-flyer award levels for next year.

Delta spokesman Paul Skrbec said the company's change to its upgrade policy comes as the airline is making major investments to upgrade amenities on those routes. That includes seats that turn into beds, improved entertainment choices and "premium dining."

He also points out that at the same time the airline was taking away upgrades on the routes to and from New York, it was adding upgrades on flights between Hawaii and Los Angeles, Salt Lake City, San Francisco and Seattle.

For those who want to remain loyal to Delta and want to fly business class, your primary option is to pay for it. A "Global Upgrade" - a bump-up available for any route - could be used, but frequent flyers say those tend to be used on more costly and longer international flights.

Flyers who switch over to another airline may have more luck getting upgrades. Other airlines have similar or even better amenities than what Delta is adding, says frequent-flyer expert, Brian Kelly, who runs ThePointsGuy.com.

Besides similar amenities on United Airlines and American, JetBlue is launching a first-class service in June called "Mint" on flights between New York and Los Angeles or San Francisco.

But other airlines still allow their high-level frequent flyers to upgrade.

While airlines routinely tweak their mileage programs, Kelly says Delta's policy change is particularly harsh because it gambles that enough people will want to pay for the seats at the expense of loyalty customers.

"It's the straw that's going to break a lot of elites' backs," he says.