American Airlines Group Inc. said it plans to change its frequent-flier plan to award miles based on ticket price, rather than distance flown, becoming the latest and largest U.S. carrier to shift its loyalty program to favor bigger spenders.

American, the nation’s top airline by traffic, said Tuesday that the changes on how its passengers earn award miles would go into effect in the second half of 2016, but it wasn’t specific about a date.

United...