The Reagan administration's decision to back off the 8 percent pay raise it promised the 2.1 million men and women in uniform this October is a choice example of how budget realities are forcing the president to restructure his proclaimed military policies.

The world of President Reagan is beginning to look far different than the world of candidate Reagan when it comes to such national security issues, all much trumpeted in the campaign, as closing the "window of vulnerability," negotiating arms control with the Soviets and, most recently, paying military people as much as they could earn in civilian jobs.

So far every idea Reagan has proposed for making land missiles less vulnerable, thus closing the window of vulnerability, has been found wanting, either by the administration or by Congress. The expense of some of the MX deployment schemes in this day of looming $100 billion deficits has always been influential, sometimes determinative.

The costs involved in a speed-up of the nuclear arms race with the Soviet Union also seem to be more sobering to President Reagan than to candidate Reagan. President Reagan has just offered to negotiate arms reductions with the Soviets even though they seem to be behaving badly in Poland, breaking the linkage formulas of candidate Reagan.

And now comes the turnabout on military pay, perhaps the most dramatic one of all, considering the fiery rhetoric Reagan and his lieutenants have blown across the country on the subject.

Candidate Reagan assailed President Carter for letting military pay fall behind civilian salaries. He brought cheers from American Legionaires gathered in Boston on Aug. 20, 1980, by promising to "implement a program of compensation and benefits for valued military personnel comparable to what is available in the private sector."

Defense Secretary Caspar W. Weinberger, in appearances before congressional committees, repeatedly has portrayed the past low pay of military people as a national disgrace. And Navy Secretary John F. Lehman Jr. as recently as last month said he would rather give up a nuclear aircraft carrier than cut into military pay.

Specifically, Lehman told Navy Times on April 18 that "We would take cuts in programs, go down to one carrier instead of two, go down to one attack submarine, rather than cutting into personnel compensation."

Despite all this talk, the administration has decided to keep asking for two nuclear carriers in the fiscal 1983 budget but halve the promised 8 percent pay raise for military people and scrap the 5 percent increase slated for government civilians in fiscal 1983, starting Oct. 1.

One reason for choosing to cut pay is that the savings on the spending side of the ledger are immediate. Canceling a weapon saves comparatively little in spending, because it is bought on the installment plan over several years. Reagan estimates that military and civilian pay cuts will save $3.9 billion in fiscal 1983.

Reagan, Weinberger and Lehman would of course have preferred to continue raising military pay. They had lived up to their promises on military pay, with this year's raise a hefty 14.3 percent, until this new reversal.

But the ailing economy is forcing them to put a "new look" on their military program, one that resembles those of the past as the nation went through feast and famine cycles in deciding how much is enough for defense.

Thus the new look is really the return of the old look.