If you are a 1928 (or later) model U.S. civil servant, the question is how is it going to work?

The "it" of course is the Reagan administration plan to overhaul the federal retirement system. One of the proposals would force people to work until age 65 to get full benefits. Currently employes can get them at age 55 with 30 years service.

On Wednesday The Washington Post reported that the president's budget, which goes to Congress later this month, would propose a major shakeup in the U.S. retirement system.

That program has 342,000 dues-paying members in this area.

Among the reported changes:

Reduced annuities for people retiring before age 65.

A rise in employe contributions to the retirement fund (now 7 percent of salary) to 9 percent in 1984 and 11 percent in 1985.

Changing the basis for computing future annuities from an employes' average high-three-year salary, to a high-five formula.

The Office of Personnel Management will not confirm (or deny) the report. But OPM Director Donald J. Devine did issue a statement which he said would calm the fears of people who are currently eligible to retire. He said in part:

" . . . I would like to make it clear that any changes which are proposed . . . will be designed to bring federal employment practices closer to private sector practics. I want to assure employes that our proposals, when they are finalized and made public, will involve reasonable steps which take into account the needs of covered employes and the commitments the government has made to its employes through the years."

Devine said the proposal "will not preclude retirement at age 55. Any proposed changes in benefits for individuals below age 55 with 30 years of service will not be implemented immediately, but would be phased in and would allow credit for the years served under the present system."

The OPM chief also said that the so-called high-three computation formula now used to determine the amount of annuity will not be changed for "at least the next three years," and that "the administration will not propose reducing benefit levels for civil service retirement. Any proposals for changing benefit levels would involve adjustment for future increases only."

That statement is reassuring. Up to a point.

It sounds as if people at or near age 55, with 30 years service, have nothing to fear. But for those who have not attained that age or service (which is about 80 percent of the federal work force) it sounds as if you might be around the office a lot longer than you had planned.