IN DECIDING to move off his earlier hands-off policy with respect to Social Security, President Reagan has gone well beyond the requirements of the moment. The administration now proposes a reduction in benefits big enough to promise not only a cure for the near-term deficit, but also a long-run surplus of size sufficient to reduce payroll taxes substantially.

Benefits now provided by Social Security are estimated, over the next 75 years, to cost 13.74 percent of covered wages, half to be paid by employers and half by workers.While the Social Security retirement and disability funds are expected to be in financial trouble for the next few years, they should then move into surplus until somewhere in the next century when, as the result of an expected upsurge in the number of elderly, deficits will reappear. Covering the total deficit expected between now and the year 2056 would require raising the total payroll tax by about 1.5 percentage points. Instead the administration proposes to cut benefits enough to produce an average yearly surplus of 2.86 percent of payroll. This is a cut equal to one-third of total benefits now payable over the next 75 years.

The two-decimal-point precision of the numbers just cited should not mislead you. Buried beneath these official estimates are a host of assumptions about the social and economic shape of the 21st century. With all their defects, these projections are needed to get some idea of changes people might want to prepare for in the future. But the degree of certainty they embody is far from sufficient to justify the sharp and immediate changes in our most basic social programs now proposed.

The major savings proposed come from a five-year phased reduction in benefits for all new retirees and an immediate and sharp cut in benefits for those choosing to retire between the ages of 62 and 65. This significant reduction in incentives for early retirement would be accompanied by an increase in incentives to postpone retirement beyond age 65, the latter produced by allowing those over 65 to draw full benefits while still working. Apart from the dubious equity of these proposals -- a subject for another day -- there are plain economic reasons for hesitation about so sudden a shift in retirement incentives.

The services of older workers may well be wanted and needed and in the economy of the future and, with better health and longer expected lives, people may want to work longer. But in recent years, with a large and -- in the near term -- probably growing number of unemployed, the trend has been strong in other direction. Over 70 percent of new retirees are now under age 65 compared with 60 percent in 1970. Studies show that early retirees are most likely to be manual and service workers with few assets and often poor health -- more likely candidates for unemployment insurance and welfare than for the vanguard of a new economic revival.

It is prudent to plan now for the work force and investment needed to support the economy of the future. But his concern should not be used as an excuse to break faith either with those depending on Social Security or with the millions more now acting in anticipation of its benefits.