Delta spokesman Anthony Black told NBC News that the change was one of many being made to the program, with others taking effect immediately.
He stressed that "most award prices will stay the same, with a greater availability of awards at the lowest prices and new 7,500 and 10,000 mile one-way awards. Members can take advantage of lower prices immediately, while other price changes won't take affect for travel more than 10 months from now."
Among the other changes listed in Delta's announcement was a new "upgrade with miles" option available only at the time of booking.
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While some changes should make it easier for passengers to access award seats, others may increase the amount of miles needed to purchase upgrades or tickets.
Travel hackers and bloggers — those frequent fliers who aggressively accumulate miles and study how to make the most of them — immediately took issue with some of the changes and the amount of information shared with customers.
"These changes may mean there are more awards available for every flight, but there will be fewer opportunities to increase the value of SkyMiles," said Scott Mackenzie of travelcodex.com.
Mackenzie states, for example, that many of the one-way awards Delta is promoting for 7,500 or 10,000 miles are likely to be available only for short or less-competitive routes.
For other flights, "Delta will now determine demand and other factors to determine how many miles are required, in the same way that it uses those factors to determine the price of a paid ticket," he said.
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Delta considers that fair.
"Today no airlines tells you that fares are going up tomorrow or where the fares may change," said Black. "We have announced that as of June 1, 2016 — more than 300 days away — the award price for some trips will change based on a variety of factors; just like they do today for revenue tickets."
That's the airline's prerogative, but Gary Leff of the View from the Wing blog is concerned customers will be flying blind when booking awards because Delta also recently removed from its website the chart that listed the set numbers of miles needed in exchange for various types of tickets.
"They say some prices will be going up and others will stay the same, and this is based on a variety of factors," said Leff. "(But) they will not share which prices for what trips will go up, and which ones will stay the same, or how consumers will even know what to expect."
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That reduction in transparency will mean Delta customers will have a hard time determining if they're getting a good value for their frequent flier miles, said travel industry analyst Henry Harteveldt of the Atmosphere Research Group. "But it may also mean more award seats are brought into the marketplace," he said. "It can work in favor of the consumer if demand is soft to a particular destination, the cheapest tickets may be put on the market for less than the minimum 25,000 points that we have today."
Changes in Delta's SkyMiles program have elicited pronouncement from travelers along the lines of "I'm leaving and will never go back," said Chris McGinnis ofTravelSkills.com. But "the value of a frequent flyer mile is dependent on demand," he noted.
"Once demand falls off, Delta will use the SkyMiles program to woo us back with added value and perks," he said. "We just have to be patient.