Southwest Airlines has started its extreme makeover of AirTran Airways' fleet of jets.

The first AirTran plane is at a contractor facility in the Seattle area this week to be converted to Southwest's use, according to executives. Highlights of the 40-day process include a new paint job outside, all-coach seating inside and a different wi-fi system.

"I realize this may be a bittersweet moment for many of you, but it's a very important milestone in our integration," Bob Jordan, a Southwest executive who has been president of AirTran operations since Southwest's buyout, said in a phone message to employees Monday.

Dallas-based Southwest last year acquired Orlando-based AirTran, the second-largest carrier in Atlanta. The merger is completed but AirTran continues to operate under its own name and colors until planes are converted and regulatory approvals are in hand.

The aircraft conversion will transform an AirTran Boeing 737-700 with 125 coach seats and 12 business class seats into a Southwest plane with 143 coach class seats.

The removal of business class -- an amenity that many AirTran frequent fliers say they'll sorely miss  -- will be the most noticeable change inside.

In addition, the all-coach cabin will have a new layout Southwest debuted last month, squeezing six more seats onto the plane with thinner seats and less recline. AirTran's seats are 18 inches wide; Southwest's seats are 17 inches wide, according to SeatGuru.com. Southwest's new interior will have seat covers made of "E-Leather," which it calls an eco-friendly "alternative to traditional leather."

Planes will also emerge with Southwest's satellite-based wi-fi, instead of AirTran's air-to-ground based wi-fi. Southwest currently charges $5 for its wi-fi, while AirTran offers Gogo wi-fi service for $4.95 to $12.95 per flight, depending on the length of the flight.

Southwest, which started flying to Atlanta on Feb. 12, plans to gradually convert AirTran planes over a period of about two years, at a rate of several planes a month.

The airline will not bring the first former AirTran plane into Southwest service until it receives a single operating certificate from the Federal Aviation Administration, which amounts to the formal go-ahead to begin combining the two carriers' flight operations. Southwest expects to get the certificate Thursday.

That means Atlanta fliers will see a growing number of orange, blue and red Southwest planes flying into Atlanta as it gradually takes over AirTran service. Southwest launched service in Atlanta with 15 daily flights, but expects to increase that as it converts AirTran planes into Southwest aircraft.

Southwest service is different from what many Atlanta fliers are familiar with on AirTran and Delta. Aside from all coach class service, Southwest also has open seating and allows two free checked bags.

AirTran has 52 Boeing 737s and 86 Boeing 717s, and all of its planes will be converted, according to Southwest. For Southwest, the conversion of the 717s also is historic: For nearly all of its 41 years, Southwest has operated an all-737 fleet.