Now that it has cut the number of people on the federal payroll, the Reagan administration is preparing for some surgery on those left behind who are benefiting from Grade Creep.

The idea is to take a look at the job people are doing to see if they are being paid too much (or too little) considering their duties.

Earlier wars on Grade Creep, with LBJ, Nixon and Carter leading the charge, resulted in a lot of noise and smoke, but not too many casualties. This time it could be very different.

Contents of a still-secret study by the Office of Personnel Management are being guarded like they were the Normandy invasion plans. But there are some leaks. This is what is leaking out:

* Grade creep, the study shows, is a bigger problem now than when the government last looked at classification errors back in the late 1970s. At that time, the Civil Service Commission (now OPM) concluded that just over 154,000 white collar federal workers (11.49 percent of the work force) were overgraded and being paid too much, while about 44,000 people (a 5 percent error rate) were undergraded for the type work they were performing.

* The new OPM study includes a broader sampling of the white-collar federal work force and it concludes that a significant number of employes in Grades 1 through 15, which include nearly the entire work force, are overgraded.

* Overgrading costs the taxpayers an extra $600 million a year, with the greatest number of overgrading errors being found in Grades 12, 13 and 14.

* Overgrading varies from agency to agency, and from region to region. The biggest problem is in metro Washington where the average white-collar worker gets around $26,000 per year.

* One of the agencies with the worst case of Grade Creep is a super-sensitive outfit here. There is evidence that many of the overgraded jobs came about because bosses helped inflate grades (to give valuable and favorite workers salary increases) to make up for low raises -- 9.1 percent in Carter's last year, 4.8 percent for Reagan's first year in office -- for white-collar civil servants.

Both OPM and the Office of Management and Budget are planning a new drive to study (desk audit) more federal jobs to see which are graded too high, too low, or just right. To be decided is whether to launch an all-out attack to study every job for fat, to limit the Grade Creep search to known problem areas (an agency, occupation or grade), or to simply limit the coming probe to audits (and downgradings where necessary) when a job is vacated.