Southwest Airlines (LUV 3.16%), which bought AirTran in May 2011, announced today that it will take the routes that AirTran operates in four cities and turn them into Southwest Airlines routes.

The changes will happen April 14 in nonstop routes servicing Charlotte, N.C.; Flint, Mich.; Rochester, N.Y.; and Portland, Maine. A limited-time fare sale is being offered to mark the change.

Southwest has already said that the AirTran operations in Branson, Mo. will change in March. AirTran operations in Allentown, Pa.: Harrisburg, Pa.; Huntsville, Ala.; Lexington, Ky.; Sarasota, Fla.; and White Plains, N.Y. were shut down in August.