Federal officials believe it will be two years before President Carter's civil service "reforms" will be fully operational, even if Congress sticks to the White House timetable and approves the major changes within the next two months.

The first big group of federal workers who will notice changes under the Carter reforms will be the several-hundred-thousand middle- and upper-level managers in Grades 13 through 18. Changes in the way they will be paid, promoted and/or punished are the first order of business.

The White House plan now calls for complete installation of new merit system pay methods for Grades 13 through 15 managers by mid-1982. When in effect, those managers will no longer be part of the automatic pay-raise cycle under which other white-collar workers benefit. Instead they will be given raises based on their performance as determined by their bosses. This sort of thing sounds good to people outside government, but it worries even the most able and hard-working bureaucrats who see it as opening the doors to added political and personal pressures from on high.

Early next year - between January and April - senior executives now in Grades 16, 17 and 18 will have a big choice to make. They will have to decide whether to stick with the status quo, maintaining job security but giving up the chance for most promotions or better jobs, or whether to enlist voluntarily in the Senior Executive Service.

Persons joining the GS-16 and above ranks after the reform bill becomes law will not have a choice. They will be put into the SES, which promises fast promotions, pay bonuses and retirement pay incentives for the successful, and demotion or dismissal for those who don't measure up to new yardsticks of performance.

The question now is, will there be a reform this year, and if so, what will it look like? The version cleared by the Senate, in an 87-to-1 vote, is very much like the package submitted by President Carter. White House aides say they could live with it nicely.

The House version of "reform" is much different. Indications are President Carter might even veto it if that is what Congress finally sends him. The House is due to begin full debate on the reform bill worked up by its Post Office-Civil Service Committee on Sept. 7.

Several of the offending (to the White House) provisions of the House reform bill may be struck shortly after debate begins. Those provisions deal with amending the Hatch Act to permit federal and postal workers to be active in partisan politics, with a new work week for federal firefighters, and with the House plan to limit the SES to a two-year trial period in three major departments, rather than allow it govermentwide. HEW and HUD are two of the agencies most often mentioned as being places for a trial run, if the House version prevails.

The White House says it still favors Hatch Act changes - cleared by the House in separate legislation but blocked in the Senate. But presidential spokesmen say it should not be part of the civil service reform package. They are opposed to adding the firefighters' work week change, since President Carter vetoed a bill to do the same thing earlier this summer.

Reform still has a long way to go. It must clear the House - with or without the amendments the White House opposes - and then be reconciled with the Senate version. Finally, President Carter must decide whether the compromise "reform" bill, even with its warts, has enough of what he wants for him to sign it.

Administration officials are most eager to get the SES setup, giving political managers greater control over the bureaucracy. Whether they want it badly enough to accept sweeteners added by unions, as reflected in the House bill, remains to be seen.