CHICAGO (CBS) – United Airlines has started moving its operations center employees into a new home — the Willis Tower, formerly known as the Sears Tower.

United, which formally combined with Continental on Oct. 1 to form the world’s largest airline, is moving the first 280 employees into the skyscraper on Monday. This is the first phase of the move of more than 2,500 people who currently work at the company’s operations center at 1200 E. Algonquin Rd. in Elk Grove Township.

United currently plans to occupy 12 floors at Willis Tower, three more floors than the company announced in August 2009.

After the move to Willis Tower is complete, United expects to employ more than 13,000 people in Chicago.

The move will also bring United’s operations employees closer to colleagues at its world headquarters at 77 W. Wacker Drive, where 630 employees currently work.

United expects to move more than 1,000 employees into Willis Tower by the end of the year. It projects that the rest of the moves will happen over the next 18 months. Eventually, Continental’s operations center in Houston will close, with those functions moved to Willis Tower. The combined airline will be run from United’s current headquarters in Chicago.

United and Continental are now both units of United Continental Holdings Inc., which will continue to run the airlines separately while it combines their operations.

United said previously that it could not afford to spend the $50 million it would take to upgrade their Elk Grove Village space.

The Elk Grove building was also the airline’s corporate headquarters until 2006, when United took advantage of tax breaks and moved into the city of Chicago and moved to the Wacker Drive building.

Shares of the United parent company slipped 29 cents to $26.15 in morning trading Monday.

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