Thousands of federal workers hit by no-fault demotion since July, 1975 would have their full pay and civil service grade restored permanently under legislation introduced yesterday by Rep. Robert N. C. Nix (D-Pa.). Nix is the chairman of the House Post Office-Civil Service Committee that will handle - and almost certainly approved - the bill.

His plan would also protect workers downgraded in the future through no fault of their own by guaranteeing that they would not have to take pay or grade cuts for as long as they remained in their jobs. Positions "redlined" for demotion would not actually be downgraded until the incumbent left them either because of a transfer, promotion, retirement or death.

Nix's proposal does not deal with jobs that might be downgraded because of various reorganizations plans of the White House.

Insiders expect, however, that the Carter administration will come up with a similar legislative package of its own to protect the job and pay status of employees whose positions are affected adversely by reorganization or consolidation of agencies. President Carter has repeatedly told federal employee groups that nobody would lose pay, rank or a job as a result of any of his reorganizations. But there is not yet any legal way to deliver on that promise.

Nix's bill concern itself with federal white-collar and blue-collar workers whose jobs are found - either by internal agency or Civil Service Commission audit - to be overgraded. Normally in those cases workers are demoted to a rank determined proper for that position. They lose their grade, but they keep their salaries for two years. (The two-year, saved-pay feature, incidentally, is the reason for the July 1, 1975, date in the Nix bill).

The committee chairman said he wants the government to act vigorously to wipe out overblown grades, which cost money and distort the job picture. But at the same time, Nix says he wants to be sure that employees due to be demoted - or those demoted since July 1, 1975, - are not punished for the classification errors of their agencies. A job would be protected provided it had been classified at its present level at least one year.

Nix's plan would simply mean that nobody hit by an agency or CSC-ordered downgrading would suffer loss of pay or grade. Instead they would keep their current rank and the salary and the future salary increases that go with it until they leave the job. Then, and only then, would the position be downgraded so the new person taking it would get the lower grade and correspondingly lower salary.

The proposal might strike some editorial writers as a boondoggle, but it makes good sense in the long haul. First, it would cut the number of costly and time-consuming adverse action grievance cases that frequently result from the no-fault demotion.

And the demotion protection should make employees more receptive to bonafide job audits.

Finally, it would make agencies less timid in wielding the grade-cutting ax since they would not be hurting people, but would be making cuts on paper that would take place later.

Future savings to the government could be tremendous - if enough grades are found to be overgraded and are cut back - with the minimum of pain to on-the-job employees, and the minimum of resistance from the bureaucracy.