The AirTran Airways frequent-flier program ends in November and be folded into Southwest's Rapid Rewards plan.
Southwest Airlines Co. on Friday said the company began notifying customers of the changes about two weeks ago.
Dallas-based Southwest bought AirTran Airways in 2011 and plans to switch all its planes to the Southwest brand by year's end.
AirTran travelers earn one credit for a coach flight and a half-credit bonus for flying business class. Reward flights cost eight credits in coach or 16 in business class.
The Southwest system is based on how much customers spend.
AirTran customers have until Nov. 1 to earn, redeem and travel on AirTran credits. Each non-expired AirTran credit on Nov. 1 will become 1,200 Southwest points.