WHAT A DIFFERENCE a year makes. At this time last year, the prognosis for the Social Security system was dire. The only question that seemed at issue then was whether the system would merely collapse by itself or drag down the entire republic with it. Now comes the annual report of the Social Security Trustees for 1983 with quite a different tone. The retirement and disability systems are said to be in robust health, and the still ailing Medicare system has gotten a brief reprieve.

The trustees report that revenues to the retirement and disability funds will be sufficient to cover benefit payments for the next 40 years even under their most pessimistic forecast of future conditions, and for 75 years if things turn out somewhat more favorably. By the end of this decade, they predict, Social Security should begin to pile up such substantial surpluses as to significantly--though temporarily--relieve pressure on other parts of the budget.

This favorable turn of events did not require the massive retrenchment from the nation's commitment to care for its elderly that some critics had deemed necessary. Partly it results from the fact that the current recovery is better than the rather dire economic forecast to which the administration had retreated from its earlier roseate projections. But most of the improvement comes from the sensible, non-revolutionary reforms that Congress put into law earlier this year.

The Medicare system is still headed for long- term insolvency. But the trust fund's life expectancy has been increased by two or three years. Now it seems likely that, without further reform, the system can cover benefits until 1990. That improvement was brought about by reforms made in last summer's tax bill and this year's Social Security measure.

This does not mean that Congress can afford to put this delicate subject on the inactive list. As the trustees note, even the relatively secure retirement and disability funds could face another short-term crisis if the economy were to repeat the experience of the last three years by falling back into another recession after a brief rebound. And the five years or so of grace that Medicare has received are barely enough time to get the needed reforms in place.

Deciding what to do about Medicare--and its companion system, Medicaid--requires answering difficult questions. As medical technology advances and longevity increases, the amount of money that the nation could spend on medical care is almost boundless. How far does the society want to go in ensuring access to that care? How shall the responsibility for paying for it be shared between the government and the individual? And if some curtailment of such spending is needed, how can it be introduced without disrupting the American medical system or destroying its excellence?

For better or worse, the Social Security system is likely to remain on the nation's political agenda. But it is reassuring to know--and important to remember in the continuing budget debate-- that the nation's largest "entitlement" program is no longer contributing to the budget problem.