Southwest Airlines, the largest discount airline in the United States, said Tuesday it will re-design the interiors of many of its planes, changing the color scheme and carpets, adding thinner, lighter seats that will allow room for an extra row to hold six more passengers.

"We are preparing now for our next 40 years with a fleet modernization plan that includes the new interior, the Boeing 737-800 which is scheduled to come online beginning in March this year, and the 737 MAX, which is expected to join the Southwest fleet in 2017," said Bob Jordan, executive vice president and chief commercial officer of Southwest.

In a press release, Southwest said the new lighter-weight seats and fixed-wing head rests offer improved ergonomics so will be more comfortable without reducing personal space. Meanwhile, the re-design will increase under-seat room for carry-on luggage.

The company said it will start retrofitting its current fleet of 372 Boeing-700s in March this year, anticipating completion in 2013, at a total estimated cost of approximately 60 million U.S. dollars. It also plans to retrofit its subsidiary AirTran's Boeing-700s and 717s.

"Other Southwest fleet types are still being evaluated for a possible retrofit," Southwest said.

Southwest, a low-cost airline based in Dallas, Texas, is the largest airline in the United States, based upon domestic passengers carried.