Copying machines in U.S. embassies from Bangkok to London have been extra busy lately reproducing an important telegram from Washington. The message alerts the 68,000 overseas feds of some potentially serious changes cooking here at headquarters city.

Unlike most State Department telegrams, this one doesn't deal with foreign policy or the latest crisis in the world. This is more serious stuff:

Seems the boss (the president) and the board of directors (Congress) are messing with the company pension plan.

Even though it takes longer for bad news to reach overseas workers, they have been hearing the same horrible things as their colleagues in Beltsville and Boise about upcoming pay and pension changes. Like their home-front counterparts, overseas feds are worried and confused.

Although State's legislative update was sent in the clear (meaning as an "unclassified outgoing telegram"), its serious tone is probably giving fits to code-breakers of foreign powers who suspect Uncle Sam is always pulling a fast one. The cryptographers must be working overtime trying to figure out the true meaning of the office chitchat message that deals with the heartburn afflicting embassy staffers worldwide.

The telegram is also being passed on to other interested overseas feds--from Agriculture Department specialists in the Far East to narcotics-watchers in Europe.

Officially, the State Department is more concerned with things like the occupation of Afghanistan, or which China is our real friend. But officials realize that employes who run the day-to-day overseas operations of our government are alarmed by what they hear from back home.

To keep the troops up to date, State's telegram gave a blow-by-blow description of the president's proposals to "reform" the federal retirement system. If Congress buys the president's plan, federal employes would have to work longer and pay more into their retirement program.The proposed retirement changes are of special interest to news-starved Foreign Service aides and other overseas types--serving in posts ranging from the plush to the putrid--because by law they must retire earlier than other feds.

Reagan's plan would require that regular federal employes take a 5 percent per year reduction in annuity for each year they are under age 65 at the time of retirement. It would also limit all retirees who are under age 62 (except those getting disability annuities or survivor benefits) to only half of the annual cost-of-living adjustment that went to retirees over age 62.

If those changes go into effect, Foreign Service types fear they might suffer big annuity reductions because they are forced to retire early:

Hoping to sooth employes, State advises: " . . . the current thinking is that if this provision is made applicable to the Foreign Service, the age under which reduced annuities would apply would be 60, and the provision would not be applicable to current employes age 50 with 20 years service . . . ."