A SHIFT IN TRADERS' APPROACH

The best investment strategies in the credit markets during 1983 may not be the same as in 1982. That seems to be the case now, with investors showing relatively little appetite for long-term Treasury bonds, even though such issues offered annualized returns of 30 percent and more last year, thanks to a combination of high yields and prices that rose as much as 25 percent.

''Markets have become far too volatile to allow participants the luxury of resting on past achievements,'' said James L. Kochan, a vice president in the fixed income research department at Merrill Lynch, Pierce, Fenner & Smith. In Bond Market Comment, the Merrill Lynch analyst added, ''Prospects appear very good that rates will drop signficantly during the first two quarters of the new year.''

The Federal Reserve is likely to encourage further declines in short-term interest rates, he said, and two key sectors of private credit demand -the mortgage market and consumer loans - may not increase significantly until the second half of the year.

Because private sector credit demand is expected to remain weak until the economy has been growing for several months, many analysts say that record borrowings by the Treasury need not push interest rates higher. Investors' Reluctance Noted

While many other economists agree with Mr. Kochan that interest rates are likely to decline - at least in the first half, if not all year - many investors are not prepared to act on such forecasts. In fact, this reluctance to buy long-term bonds has been evident since early November, and it was not changed by the Federal Reserve's actions to reduce the discount rate charged on loans to financial instititions in mid-November and mid-December.

Since early November, prices of long-term Treasury bonds have dropped about a point or two, so that the 10 3/8 percent issue due in 2012 was recently offered at about 99 to yield 10.44 percent. According to traders and other market analysts, another cut in the discount rate might not provide much stimulus to prices.