Once known as an upstart carrier, Southwest Airlines recently launched an ad campaign that has some airline industry experts wondering whether it signals a new direction — and new fees — at the nation’s largest domestic airline.

The new ads, which began airing during the television broadcast of the NCAA tournament, don’t mention the airline’s most popular appeal — that your first two checked bags fly free and you don’t pay to change flights. The ads also don’t feature funny dialogue that slams competitors — a common theme in previous Southwest commercials.

Instead, the latest ads are polished and professional and push the message that Southwest is the airline for entrepreneurs and pioneers. “We’re never finished, never satisfied and we never stop looking for a better way,” the ad’s narrator says.

Could it be that Southwest is preparing the public for a big change, such as the introduction of fees to check bags or change reservations?

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“They may be changing their tune because maybe something is coming down the pipeline,” said Anne Banas, executive editor for the consumer travel site Smartertravel.com. She said she would not be surprised to see Southwest add bag fees, which generate millions of dollars in revenue for other carriers.

Not so, says Southwest. The new ads are designed to emphasize other qualities besides the carrier’s free bags policy, airline spokeswoman Whitney Eichinger said.

“It’s more about showcasing who Southwest is now,” she said. “We don’t have a plan to charge for bags.”

In a related development, Southwest last month connected its booking system with AirTran Airways, the low-cost airline bought by Southwest in 2010. Fliers can now book flights on either airline for service starting April 14 in one transaction.

But passengers flying AirTran must still pay fees to check bags and to change reservations.