An estimated 50,000 to 70,000 federal workers are expected to retire over the next two months, to take advantage of a cost-of-living benefit that Congress is preparing to wipe out after hundreds of lame-duck Senate and House members become eligible for it.

Massive retirements on that scale -- normally about 100,000 U.S. workers retire in a 12-month period -- would create thousands of new job openings in government. It could also produce enough vacancies to prompt President-elect Reagan to modify his announced plans to freeze federal hiring in January in order to reduce the size of the bureaucracy.

In a move that will raise the lifetime pensions of about-to-retire legislators (thousands of dollars a year in some cases), Senate and House conferees on the budget reconciliation bill have extended through at least January the time period when any member or federal worker can retire and still take advantage of a 7.7 percent cost of living adjustment that took effect Sept. 1.

Congress had originally intended to abolish the so-called "look back" provision effective on the date the legislation was signed into law by the president. That would have been sometime this month, or early December. But lame-duck legislators persuaded their colleagues handling the budget reconciliation bill to extend eligibility for the look-back benefit so they could cash in on it. Conferees have agreed to allow eligibility for the look-back benefit to extend 45 days AFTER the president signs it.

The change would give members of Congress who plan to retire in January, either voluntarily or because they lost reelection bids, time to qualify for the look-back benefit before it is erased permanently.

There is also a plan brewing in Congress, reported here yesterday, to lift the lid on frozen executive federal pay levels. That would raise the salary of members of Congress by more than $10,000. Such an increase, if Congress has the political nerve (or gall) to go through with it, would raise the average salary of about-to-retire members, and that would further increase the pensions due them.

Retirement changes approved by the conferees in these closing days of Congress must still be approved by the full House and Senate and signed into law by the president. If the 45-day extension of time to qualify for the look-back benefit becomes law, federal officials estimate that retirements in December and January would soar to perhaps record levels.

December and January have always been popular months for federal workers to retire. There are tax advantages for most if they can retire in January or February. In December 1979, 10,015 federal and postal employes retired. In January of 1980, 16,372 retired.

The upsurge in retirements at the end of the year could also strain federal personnel and processing offices, causing longer-than-normal delays in getting first pension payments to new retirees. Other retirement actions approved by Senate-House budget conferees would:

Guarantee that federal, postal and military retirees continue to get two cost of living raises each year.Those inflation catchups now come on March 1 and Sept. 1 and retirees get the extra money in checks mailed the following month. The Carter administration sought to limit U.S. retirees (such as persons on social security) to a single COL adjustments each year. But heavy pressure from federal and postal unions, and the fast-growing National Association of Retired Federal Employes, blew that plan out of the water. Candidate Reagan promised NARFE leaders that he would retain the twice-yearly COL system, arguing that inflation is the villain, rather than the end result of the regular six-month adjustments.

Eliminate a "look-forward" feature of current federal retirement law which permits persons retiring BEFORE a COL takes effect to get all of it as if they had been retired for a full six month period.