Some U.S. aides who are supposed to point out any warts on President Reagan's plan to change federal pay-promotion-firing rules believe they are caught between a rock and a hard place.

The administration wants to revamp the system Uncle Sam uses to determine which of his 1.4 million white collar workers--300,000 of them here--are worthy of raises and promotions, and who gets laid off during reductions-in-force.

Comments on the proposed rule-making must be at the Office of Personnel Management by May 31. Administration officials hope to have at least some of the rule changes in effect by October.

But a number of career officials in the personnel community say they get the impression that nothing short of wholehearted support for the administration proposals is welcome.

The pending rule changes would set up a uniform, government-wide job rating system. An employes' eligibility for within-grade pay raises and status during RIFs would be based on performance rather than seniority. The White House contends that the new system would reward the best workers and give greater protection from RIFs to women and minorities, who frequently are laid off because they lack seniority.

All federal unions and many employes oppose the changes. They fear that the new system would be manipulated by political appointees to deny career people raises and get rid of them during RIFs.

Most critics of the plan argue that the administration is only interested in comments that are supportive of the changes.

Patrick Korten, OPM's assistant director for policy and communications, says he has heard the same thing "and it is flat out, absolutely not true.

"We've sent letters, we've told people orally that comments are essential and we want as many as possible . . . . We really mean it."

Korten said, "There are a couple of personnel directors who don't really believe it that OPM wants comments and criticisms alike and I've been doing my darnedest to persuade them otherwise. The other day I had to chew out an assistant secretary of a cabinet department and tell him to go down and beat on the personnel director" to stop putting out the word that critical comments are unwanted.

"Obviously we are committed to the direction" of the proposed rules, Korten said, "but we need comments. . . . We want to know what problems people foresee."

If you were a doubter who is now reassured, you can send your comments to John W. Fossum, assistant director for performance management, Room 7520, OPM, 1900 E St. NW 20415.