Southwest Airlines as we know it is over. Here's what's changing, from bag fees to basic economy

Southwest Airlines announced Tuesday what was once unthinkable: It will start charging customers to check their luggage.

It's a $300 million gamble. Last year, Southwest said its "rigorous research" found it would lose that much in market share if it started charging bag fees. The policy has set Southwest apart from its competitors for decades.

Getting rid of its famous "two bags fly free" strategy is part of a massive push at the carrier to ditch its longstanding customer perks and policies. Southwest also announced last year that it's moving from open seating to a single-class cabin in order to raise revenue. Another change announced Tuesday: basic economy tickets that don't allow free changes.

Here's what travelers need to know about the changes:

Travelers who buy any ticket except Southwest's top-level Business Select fare will have to pay fees to check bags. Customers who purchase a Business Select ticket will be able to check two bags for free.

Top-tier A-List Preferred frequent flyer program members will also get two checked bags for free. A-List level members will be able to check one bag for free, as will those with a Southwest credit card.

Southwest didn't disclose how much it will cost to check a bag but fees start at $35 apiece on competitors Delta, United and American.

Southwest has been under increasing pressure to raise revenue and improve returns after activist hedge fund Elliott Investment Management took a stake in the airline last year and pushed for changes to the carrier's business model.

Southwest executives have long told Wall Street investors and customers that its "two bags fly free" policy is sacrosanct, something that sets it apart from competitors.

At an investor day in September, the carrier said it would make up to about $1.5 billion if it charged for bags but lose $1.8 billion in market share from the policy change.

Lots! Southwest is going to launch a basic economy fare that is not refundable and doesn't allow for changes. It won't allow for same-day standby tickets.

Flight credits for those "basic" tickets, if unused, will expire in six months while credits for other flights will expire in 12 months. Previously, Southwest credits didn't expire.

The carrier last year said it plans to start selling tickets with assigned seats, ending its decades of its open-seating policy. It will also soon offer seats with extra legroom, a bid to compete with more full-service airlines.