Federal agencies today will be told how they can put a legal freeze for up to two years on downgrading actions that threaten the job and pay grades of an estimated 3 of every 10 civil servants.

New guidelines from the Civil Service Commission will give agencies the same option that CSC earlier extended to the Department of Health, Education and Welfare.HEW faced the obore of chopping the grades of up to 37 per cent of its giant work force. It was given until Dec. 31, 1979, to defer the demotion actions that would have been triggered by a combination of reorganization and the discovery of widespread and long-standing classification errors in many agencies. White collar government workers are ranked, for pay and status purposes, in 18 grades.

Normally agencies are supposed to make regular audits of each employee's job to determine whether he or she is doing the work to justify the grade. When it is discovered that an employee is overgraded, the agency is supposed to demote the employee within 90 days, even if the overgrading (as is usually the case) error was made by the agency. Employees who are demoted keep their salaries for two years, in most cases, then revert to the lower pay level of their new grade.

In recent years agencies have been pushing hard to ferret out overgrading errors. That, combined with the coming crunch triggered by the Carter reorganization, means that hundreds of thousands of workers can expect demotions in 1978. When HEW Secretary Joseph Califano Jr., discovered the magnitude ofagency, he complained that it would be too tough on employees, hamper reorganization and HEW's functions if the whacks had to be made all at once.

CSC, which has the final say on federal job classification matters, gave HEW permission to defer downgradings until the end of 1979, with the understanding that it would get its personnel house in order during that time.

Last June a group of personnel directors from big and small government agencies demanded the same moratorium on downgradings in their agency but were turned down by CSC. They lobbied hard, arguing that massive downgradings would be unfair to workers and hurt reorganization. On Nov. 17. this column reported that CSC was working on guidelines that would give other federal departments the same grace period given HEW. Agencietoday.

(The President already has proposed that any federal employee hit by "no-fault" demotion during 1977 be restored to his or her old grade. A House committee is working on legislation that would cancel most 1977 demotions - except for disciplinary reasons - and it is expected to become law this year.).

In order to get permission to eet the following guidelines and get CSC's approval. They say: