WHY WOULD Moscow change the pattern and test-fire two (unarmed) missiles toward Hawaii? One could have, though it didn't, flown over American territory -- which would have been a first. The two could have, though they didn't, ''bracketed'' Hawaii -- the method of zeroing in on a chosen target. It conveys a chill to declare -- as the Soviets did in a routine testing announcement -- that missiles are going to be aimed closer and more directly at American territory than in any previous tests. An extra puzzlement is added when the event takes place during a time that the two superpowers are negotiating a missile agreement. After the Soviet announcement, the United States protested, and for whatever reason, Moscow slightly altered its plans. Still, the Soviets cannot be unaware of the symbolism of such conduct and of its political reverberations in the United States. They did nothing ''illegal,'' but they violated the unwritten code of nuclear manners that ought to hold in these affairs.

It is not only nuclear manners that need to be observed by the great powers. Take Soviet conduct toward Sweden, a neutral, friendly and unoffending country into whose territorial waters Moscow has been running submarines for years. A private Swedish analyst, Milton Leitenberg, reported yesterday that these incursions, which are measured in the hundreds, have been ''enormously persistent,'' have been conducted with ''fantastic effrontery'' and continue. His report happened to come just as Mikhail Gorbachev proposed that NATO and the Warsaw Pact negotiate limitations on military activity in the seas -- the international waters -- of Northern Europe.

There are important matters to be negotiated between the two powers. But there are important steps that each power should be expected to undertake on its own, without negotiation and without the compensation assumed to be available in a negotiation. The Kremlin can begin by apologizing for aiming missiles into waters near Hawaii, and it can stop its menacing of the Swede