A changing of the guard always offers an opportunity for renewed care and effectiveness. This opportunity is heightened when an appointment as auspicious as that of Martin Feldstein is made to head the President's Council of Economic Advisers. The need is not to miss the opportunity either through the administration's failure to make use of Professor Feldstein's professionalism - or the professor's failure to maintain that professionalism in testing his present points of agreement and disagreement with Reagan policies.
Mr. Feldstein's starting point is, as one of his Harvard colleagues said, a level of capability ''vastly higher'' than the administration has had in the economic field so far. Even allowing for ivy-covered loyalty, here is the kind of ratification sought by the White House. According to officials, the appointment was intended to have professional respectability without too much ideology.
Such an approach is extremely important now when economic policies seem so much influenced by political attitudes. The public wants assurance that the President is receiving and listening to economic advice from his economic advisers. It wants reports and forecasts from those advisers that are not immediately open to criticism as designed for political palatability. The impression persists that the just-resigned chairman of the advisers, Murray Weidenbaum, considered some administration forecasts too rosy. Are they too rosy? Mr. Feldstein will have performed a major service simply by ensuring credibility for White House facts on the economy.
By basing policy on economic facts, Mr. Feldstein and the White House would be in the position of adjusting policies to fit circumstances rather than ideology. This process may already be underway, as conspicuous flat-out supply-siders leave the administration. And, indeed, as Mr. Reagan himself switches from supply-side tax-cutting as the universal panacea to steps like support of his party's huge tax-increase package to reduce deficits.
Mr. Feldstein is also a supply-sider, but - as a former chairman of the Council of Economic Advisers, Herbert Stein, has said - he is not a ''supply-side kook.'' He is not of the radical ''new right''; he is rather in the ''neoconservative'' camp, with his name on the masthead of the neoconservatives' Public Interest magazine, for example. In his public utterances, he has:
Looked for interest rates to come down under the impact of a continued slow economy, reduced inflation, and steady Federal Reserve policy; welcomed the financial crisis of the social security system as drawing attention to its deficiencies; suggested excluding half of interest and dividend income to encourage saving and investing; favored the full tax leasing provision (permitting in effect the buying of tax losses) which is cut back in the current tax-hike bill; and called for the postponement but not elimination of President Reagan's 1983 tax cut.
The Feldstein thrust is obviously to bolster capital investment to increase productivity and improve the economy through trickle-down effects. Is he open to consideration of recent economic thinking that sees productivity in terms of investing not only in plant equipment but in the human capital of America's people? In terms of investing in more labor and less plant in some instances, thus providing more jobs rather than fewer and fewer?
Whatever the answer to such questions, the approach to them will be the better for continuing on what seems the administration's new tack: giving more weight to actuality and less to ideology.