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It will be sad to watch the demolition of the Jewish Community Center where so many San Franciscans spent their after-school and weekend hours. It was a beautiful building, but "was" is the key word here.

Not every old building is worth saving, especially in cities like San Francisco and New York, where undeveloped land for new construction is nonexistent. If we retain everything that is old, we will never give today's architects the opportunity to build the marvelous edifices of the future.

In the case of a JCC, there is an even stronger reason for a new facility. With modern health clubs popping up all over, a JCC can only be competitive if it has a modern building with the latest in exercise equipment and a modern swimming pool.

What's more, to succeed financially it must have such revenue-generating features as a restaurant and Judaica store.

That's just what the board of the JCC of S.F. proposes for its new facility. Working with two local architectural firms, the board has come up with a design for a building that will serve as a magnet to old and new members.

The board's success in raising money for the project is testimony to the quality of the design, which includes a glass-roofed atrium, a six-lane lap pool and an underground garage. Although the design was just made public this week, the JCC has already raised $45.5 million of the $70 million it will cost to build the facility, increase staff, relocate from the old building and endow the future of the new facility.

Some of San Francisco's most prominent Jewish philanthropic families are making gifts. These are the same families whose members used the current facility over its 66-year history. They know the old building. And they know it can't be adapted to the year 2000 and beyond.

We urge Bay Area Jews to support this project and make a new JCC in San Francisco a reality. Millions of dollars must still be raised. The generosity of us all is required.