W. Europe plans for loss of US missiles. ON EVE OF WASHINGTON SUMMIT

A dramatic change is brewing in Western Europe's strategic debate: from how to live with the presence of American nuclear missiles, to how to live without them. The debate officially began yesterday as NATO ministers meeting in Brussels began two days of talks on how to improve the West's conventional forces after US medium-range nuclear missiles are eliminated.

Ministers from 14 alliance member states gathered at NATO headquarters to review progress in strengthening the alliance's ability to resist a sudden land and air offensive by the Soviet-led Warsaw Pact.

Diplomats say the meeting of NATO's Defense Planning Committee takes place at a critical time for NATO as it strives to map out an overall strategy on security and arms control following months of disarray over the medium-range missile question.

Forcing the shift from living with US nuclear missiles to living without these weapons is the approach of next week's superpower summit in Washington. There, President Reagan and Soviet leader Mikhail Gorbachev plan to sign a pledge to withdraw their intermediate-range (INF) nuclear missiles from the European continent.

Additionally, some West European politicians suspect, the two leaders may make a start on considering long-term deep cuts in their transcontinental ``strategic'' nuclear forces.

On the surface, all this would seem wonderful news for the West Europeans. In fact, the impending ``Euromissile'' treaty is causing not only celebration, but also concern.

The celebration is genuine. For nearly a decade, Washington's NATO allies have been groping for a politically palatable response to the appearance of an unprecedentedly powerful array of new, triple-headed Soviet rockets. Called SS-20's, these mobile intermediate-range weapons are capable of reaching the farthest corners of Western Europe, reinforcing the Soviet bloc's conventional-force superiority on Europe's East-West dividing line.

A key element to the NATO response was deployment of new American arms in Europe - Pershing and cruise missiles posing a similar threat to Soviet soil. The reasoning was that this would prod the Soviets to negotiate a European disarmament deal.

The NATO missile deployment was not easy for West European governments, since it sparked widespread street protests, happily cheered on by the Soviets.

``This is a political victory for the West, and for NATO unity,'' said former British Foreign Secretary David Owen in an interview with The Christian Science Monitor. ``Certainly, we in Europe will want to see it [the treaty] ratified in the Senate.''

A leading European arms expert added, ``Fundamentally, most Europeans are satisfied, and feel that non-ratification would by very bad.''

But both these men - and other officials, diplomats, and analysts - also said the approaching arms deal raised issues of concern obscured during the long search for the treaty. Among the issues singled out are the following:

The conventional balance in Europe. The Soviet bloc retains a hefty edge over NATO forces in this area, with more troops and tanks. Although next week's treaty will leave US aircraft and submarine-launched missiles in the European arena, the withdrawal of Pershings and cruises is seen as removing a major NATO nuclear deterrent.

``Decoupling.'' This refers to the danger that Western Europe would be left by the Americans to fend for itself in a European war. The reason West European governments wanted new US missiles to balance the SS-20s in Europe in the first place was a fear that in the event of a a Soviet attack, America's only militarily workable response would be to unleash its nuclear forces. And would a US president risk Chicago for Bonn? European strategists wondered, and still do.

The cost of defense. Nuclear weapons such as Pershings are a cheaper form of defense than are conventional armies. To maintain an East-West balance after the missile treaty will be costly, at a time when worldwide stock-market plunges have made the US budget deficit front-page news. European governments, and nongovernment analysts, say one effect of the treaty will be a need for European NATO members to spend more money on defense, and possibly to work out some reduction in US force levels in Europe along the way.