A major change in the Social Security law will reduce some so-called windfall benefits for many people now working for Uncle Sam.

The change is designed to reduce the relatively high Social Security payments that U.S. employes can get by working briefly under the Social Security system, although most of their careers were spent in government jobs not covered by Social Security or subject to Social Security taxes.

Exempted from any benefit reduction are persons now age 60 or older and those eligible (or who will become eligible) for Social Security benefits before Dec. 31, 1985. Also exempted are federal workers hired after next Jan. 1 (they will be covered by Social Security and the federal retirement system) and workers with more than 30 years of service in a Social Security-covered job.

What all of the above means is that the anti-windfall changes will affect most people who are now in government service who are (or who may be) eligible for Social Security in the future.

Under the current system, Social Security benefit formulas are the most generous for people who have relatively low lifetime earnings under Social Security-covered employment. The assumption is that most people spend the majority of their work lives in jobs covered by Social Security and that those with the lowest earnings are the ones who need Social Security benefits most.

Benefits for those so-called low-earners (or persons who work under Social Security just long enough to qualify for benefits) are calculated to give them the maximum payment possible. Some persons in that category get a monthly Social Security benefit equal to 53 percent of their monthly earnings under Social Security-covered jobs.

But the benefit percentage for high-earners (and those who put in the most time under Social Security) is scaled down. For some--the people who made the most or worked the longest under Social Security--their benefit is calculated at the rate of 28 percent of their earning level.

The changes, affecting future U.S. retirees who will also be eligible for Social Security, reduce the formula used to compute their Social Security benefit.

Instead of getting benefits equal to 90 percent of the first $254 in average monthly earnings under Social Security, the formula will gradually be reduced to 40 percent of earnings by 1990.