Federal and postal workers who retire next month, when Congress is expected to deliberalize U.S. retirement rules, will lose anywhere from $150 to $1,500 a year in pension benefits. But don't jump out the window or tear up your retirement papers until you finish reading this. Here is the story:

Senate-House conferes (meeting on the giant budget reconciliation bill) have voted, as one money-saving part of that bill, to make significant changes in government retirement rules. Of immediate interest to the 200,000 retirement-age U.S. workers is the plan to drop the "look-back" benefit of retirement law.

The look-back provision allows individuals to retire and get the benefit of the cost-of-living (COL) raise that went into effect just before they retired. The most recent COL adjustment (effective last Sept. 1) was for 7.7 percent.

Under current law persons could retire any time between the last COL and the next one (due March 1 unless Congress kills it) and get an annuity based either on the 7.7 percent COL raise, or on any extra benefit they earned through longer service. They can have the best of two worlds, the highest annuity computation based on longer service, or the last COL, whichever is bigger.

Congress and the Carter administration want to change that feature, which is not available to most private industry workers. To do it they have inserted language in the budget reconciliation bill that would kill look-back, effective on enactment -- when the president actually signs the entire package. e

That package is not fully wrapped. Congress left for the preelection recess before finishing all work on the budget bill. It is not due back in town until Nov. 12, after the election. That means the retirement law cannot be changed until then. And in all liklihood it will take several days once Congress reconvenes to clear the measure in conference, send it back to the full House for a vote and then through Senate for a vote. Once all that is done, the entire budget reconciliation package, with the look-back eliminating feature, goes to the president. It becomes law when he signs it.

People who retire BEFORE the look-back provision is wiped out will benefit from it. That is, they will be able to have their annuities boosted by either the 7.7 percent amount, or the amount of the annuity increased because they worked longer.

People who retire AFTER the look-back feature is dropped (assuming that happens) will not be able to get the 7.7 percent benefit.

There is always a chance the lame-duck Congress will not act on the budget reconciliation bill at all. Or that it will delay action for weeks. The point is that the look-back provision (while in grave danger) has not been officially eliminated, and will not be until President Carter signs the bill. o

If you are eligible to retire you may want to consider whether to do it soon to get benefit of the look-back. Many people will do it, if Congress changes the law. Others, for a variety of family, financial or personal reasons, will decide to work longer. Nobody can tell you what to do. But you should be aware of the likelihood that a major retirement benifit is up for change, and once the change is official you will not be able to recapture it.

Nothing can or will happen until Congress comes back to town and takes up work on the reconciliation bill. But once they are at work, you may have a major career decision to make.