''We built in enough redundancy to allow entire portions of floors to be removed without affecting the building's structural integrity, on the assumption that someone might need double-height floors,'' said Larry Silverstein, president of the company. ''Sure enough, Salomon had that need.

''And there were many other ways that we designed as much adaptability as possible into the building because we knew that flexible layout is important to large space users.''

Nearly 2,000 people will be working on the retrofit project during the peak period. The cost, which is estimated at $200 million - not including carpeting, furniture and other office equipment - will come out of Salomon's pocket.

''We made a landlord contribution to the work,'' Mr. Silverstein said, ''but Salomon's costs will go well beyond that contribution by many, many times.''

MORE than 375 tons of steel - requiring 12 miles of welding - will be installed to reinforce floors for Salomon's extra equipment. Sections of the existing stone facade and steel bracing will be temporarily removed so that workers using a roof crane can hoist nine diesel generators onto the tower's fifth floor, where they will become the core of a back-up power station.

To help shuttle Salomon employees between floors, construction crews are adding two escalators and four elevators inside the tower. And to help adjust the floor layouts to Salomon's needs, workers are moving sections of the tower's ''core'' area, which includes pipes up to two feet in diameter and air-handling equipment the size of delivery trucks.

''This is the first time I've every seen such dramatic interior changes being made in a new building,'' said Irwin G. Cantor, structural engineer for the project. ''And the whole world is watching.''