President Reagan's plan to revamp the federal pension plan would trim lifetime benefits by more than $100,000 for the typical future retiree, according to a new report by the General Accounting Office.

GAO studied the impact of the proposed pension "reforms" in the president's budget at the request of Rep. Mary Rose Oakar (D-Ohio). She opposes the changes.

Congress -- which has rejected the president's proposals for the past five years -- would have to approve each of the money- saving changes before they could be applied to the retirement program. It covers nearly 3 million active-duty civilians and more than 2 million retirees. That includes 347,000 workers in the Washington area and about 75,000 retirees.

The president said he wants to cut the cost of the federal retirement system by making it more like private pension plans. His proposals would raise the government's retirement age from 55 to 62, raise employe retirement contributions from 7 percent to 9 percent of salary, trim future cost-of-living adjustments (COLAs) for retirees, and use a less-generous formula to calculate starting benefits for new retirees.

Currently, employes may retire without penalty at age 55 after 30 years of service. Benefits are based on service time and the highest three-year salary average. The president would base pensions on length of service and the highest five-year salary. Anyone retiring before age 62 would have to take a reduced pension.

In calculating the impact of the proposals, GAO used as its model a federal worker retiring at age 61 after 30 years of service, with a beginning pension of $15,000 and a life expectancy of 19 years. Using that model, GAO concluded that:

*If the president's plan became law, that "typical" retiree would lose $428 per year because of the change in the three-year-high retirement formula to a five-year-high system.

*The typical employe retiring at age 61 would take a 2 percent (or $300 per year) reduction in initial pension. Under the president's plan, anyone retiring in the future before age 62 would be subject to a 2 percent penalty per year. An employe retiring at 55 would be hit with a 14 percent pension reduction.

*Limiting future COLAs for retirees to 2 percentage points less than the actual rise in living costs would, GAO said, trim another $300 per year from an employe's pension if the inflation rate averaged 5 percent a year.

*Requiring workers to pay more for pension benefits (most private pension plans are employer-financed) would, GAO said, mean a cut in take-home pay for current workers. It estimated that an employe earning $27,000 with 14 years of service would pay an additional $11,751 into the retirement fund if the change was made. Insurance Refunds

Feds and retirees due 1985 health insurance premium refunds should get them within the next few weeks. On Friday the president signed legislation allowing retirees to get the same refunds as workers.

More than 350,000 people in the Washington-Baltimore area are due the refunds, which range from $17 to almost $400. Amounts depend on the plan and coverage that the recipients had last year. Eligible individuals will be notified by letter of the amounts due them and will be asked to verify their addresses.

Plans offering refunds are Blue Cross-Blue Shield, Aetna, American Federation of Government Employees, Foreign Service plan, Government Employees Hospital Association, Government Employees Benefit Association and the National Association of Letter Carriers.