Several hundred thousand federal employees who could have, and probably should have, retired last year or could have switched to the new Federal Employees Retirement System (FERS) missed their deadline because of surprise last-minute congressional changes in pension rules.

They missed out because Congress made important changes in their pension program while many workers were on Christmas leave. The wounded include high-paid executives, low-paid longtime workers and many who are now eligible to retire.

Many pension experts estimate that four of every 10 federal employees would be better off under FERS than by staying in the old Civil Service Retirement System. But fewer than 3 percent of those eligible switched by the Dec. 31 deadline. Some didn't understand or trust FERS. But many more were waiting to see what, if any, changes Congress would make in retirement rules. For many, the changes are good. But they came too late.

In the race to come up with a budget package, Senate-House conferees approved legislation covering everything from a controversial $8 million appropriation for religious schools in France to a budget cut for a U.S. agency that refused to give a Texas congressman's female traveling companion an airplane ride. Some things in that Christmas package are still being discovered.

Here's what happened for federal employees:On Dec. 21, the Monday before Christmas, Congress changed pension offset rules for people who joined FERS by Dec. 31. The rules affect anyone who gets a federal or public pension and who applies for an (unearned) spousal or widow benefit under Social Security. The change meant that federal workers who switched to FERS by the 31st could avoid the offset that reduces Social Security benefits those employees would get. This column reported the change the next day. But many workers on vacation, plus most outside this area, didn't get the word. Neither did federal retirement or Social Security offices that were supposed to be advising workers.

The same day, Congress changed lump sum pension payment rules for persons retiring after Jan. 2. Workers who read of the change here on Dec. 22 barely had time to put in their retirement papers to qualify for a full lump sum pension payment. Those who missed the deadline and who retire this year will get only 60 percent of their lump sum payment in 1988, and the remainder in 1989.

The next day, Congress ruled that employees in the FERS pension program will be able to contribute up to 10 percent of their pay to a tax-deferred program and get a 5 percent matching contribution from the government. Had Congress not exempted the federal plan from antidiscrimination rules covering similar plans in industry, high-paid ($50,000 per year and more) employees might have had their investment options trimmed to as little as 3 percent. Uncertainty over the amount they could invest kept many highly paid workers from moving into FERS, and the exemption they craved came too late -- Dec. 22 -- for many of them to make the Dec. 31 deadline to switch to FERS.

The lesson many federal workers learned, too late, is don't leave town until Congress does.