The nature and shape of the global ''arms race'' is changing. According to the respected International Institute for Strategic Studies in London:
* Although the world keeps on spending more and more on weapons (an estimated From now on, many nations simply won't be able to afford to spend as much as before on adding to their arsenals. The emphasis is switching to boosting quality instead.
This means (1) that arms control agreements limiting new refinements are more urgent and crucial than ever, but are more difficult to achieve. And (2) that it is even more important for the West to stop the flow of high technology to Warsaw-bloc nations.
* Arms sellers are competing even harder to sell to the third world.
* As populations grow older and birthrates fall, soldiers, sailors, and airmen will age and manpower pools will be smaller. One result: More women are already appearing in backup posts, especially in the United States.
* The Soviet Union will have to rely on Muslim and central Asian recruits for as much as one-third of its armed forces by 1990. NATO allies in Europe will have to turn more and more to Mediterranean countries for reserves.
These points are made by the institute in two ways - in its latest (1983-84) edition of ''The Military Balance,'' an annual compilation and review of world armaments; and in comments on the edition by its director, Robert O'Neill.
Dr. O'Neill and his staff see no dramatic shift in the overall balance of forces between NATO and the Warsaw Pact, though the report does conclude that ''the numerical balance over the past 20 years has slowly but steadily moved in favor of the East.''
''At the same time,'' the report adds, ''the West has largely lost the technological edge which allowed NATO to believe that quality could substitute for numbers.''
Nonetheless, the institute sticks to its conclusion that neither side is strong enough to attack with impunity.
The report breaks new ground this year by adding sections on population trends and the world economy.
So far nations around the world have continued spending heavily on arms. Although global gross domestic product growth amounted to only 1.3-1.5 percent between 1981 and 1982, defense spending appears to have jumped by 10 percent.
If the US and the USSR are excluded, NATO and the Warsaw Pact have held about steady in their spending in the five years to 1982. But both superpowers have boosted their defense budgets.
The highest growth rate in arms spending has been in the Middle East, where spending shot up by more than 35 percent between 1978 and 1982.
''But where is the money to come from for this kind of growth in the coming years?'' asks Dr. O'Neill. ''With recession and domestic needs, how can societies keep on spending so fast on defense? It will be increasingly difficult for countries to keep on devoting so much of their budgets to defense. . . .''
Thus the institute spotlights a trend that has begun to worry a number of governments.
So far the Reagan administration has boosted US defense spending considerably , but less wealthy countries such as Britain are finding it harder. Debate continues in London on whether Britain can really afford up to (STR)10 billion (about $15 billion) on the Trident submarine missile to be bought from the US and installed on four British-built nuclear submarines.
Dr. O'Neill warned that the Soviet Union might be more resilient and willing to suffer sacrifices to keep defenses high than some in the West predict. ''Trying to spend the USSR into collapse is a dangerous policy,'' he said.
Looking at population trends, the institute is concerned about West Germany in particular, whose available manpower for the military is expected to fall by 50 percent by 1999 (from 6.5 million to 4.2 million).
''It will be necessary,'' Dr. O'Neill says of West Germany, ''to go outside the 17-30 age group, and it may be necessary to decrease medical and educational standards of entry. That could be a problem in an era when the skills needed to operate new technological weapons are increasing. . . .''
Already the US has begun to shift large numbers of armed services women from administrative and medical duties - where 54 percent of them were employed in 1971 - to communications, logistics, and equipment repair. Only 35 percent of women were in administrative and medical areas by 1981.
The latest report also looks at the age of NATO and Warsaw Pact ships, and concludes that the Warsaw Pact has a small but useful advantage.
In the US Navy, 23.9 percent of vessels of more than 100 tons were at least 20 years old (100 out of 419). The figure for NATO as a whole: 32.6 percent (447 out of 1,373).
For the USSR, 30.7 percent were over-age (458 out of 1,493). But for the Warsaw Pact as a whole the figure was 28.9 percent (499 out of 1,723).