DALLAS — Delta is providing more details about changes to its frequent-flier program after complaints about the overhaul from some travelers.

Delta is changing its program so that starting in 2015, customers will earn miles based on how much they spend, not just miles flown.

On Thursday, the airline released charts showing how many miles you’ll need to earn a free trip. That’s a key piece of information for SkyMiles customers that Delta had planned to hold back until late this year.

Delta is leaving the minimum number of miles needed for a U.S. trip at 25,000 miles and lowering requirements on some flights, especially international ones in business and first class.

The the airline decided to release the miles-needed charts now instead of in the fourth quarter because “customers were asking for greater transparency,” Jeff Robertson, Delta’s vice president of loyalty programs, said in an interview.

The reaction among loyalty-program experts was mildly positive.

Brian Kelly, founder of travel website ThePointsGuy.com, said the redemption rates were good news, although much will depend on how many seats Delta makes available for people cashing in miles.

Jonathan Wu of the consumer-finance website ValuePenguin.com said the new redemption rates “look like minor tweaks.” Even though Delta didn’t broadly raise mileage requirements, the overall changes in SkyMiles will still make the program less valuable for leisure travelers, he said.

Last week, Delta announced that starting next Jan. 1, it will base miles earned on how much passengers spend — a big advantage for business travelers and others who buy expensive tickets. Robertson acknowledged that leisure travelers who won’t fare as well “shared their disappointment with us.”

Loyalty programs at Southwest, JetBlue and Virgin America are already based on spending, not miles.

www.delta.com/content/dam/delta-www/skymiles/2015-program/2015SMP-award.pdf