Or just a different messenger?

The expected appointment of Robert J. Brown as U.S. ambasador to South Africa could turn out to be anything from a public relations fizzle to a diplomatic breakthrough.

It all depends on whether President Reagan has in mind to send Pretoria a new message, or only a new messenger.

If the latter -- if the Reagan administration hopes to make its basic South Africa policy more palatable at home by making the former Nixon aide the first black U.S. ambassador to that country -- it is virtually certain to flop. And maybe worse than that.

But if the appointment of the 51-year-old North Carolinian is the president's way of underscoring a shift in policy, it could turn out to be a stroke of diplomatic genius.

Since late last month, officials have been "rethinking" the administration's South Africa policy, apparently looking for a way to avoid tough new sanctions against the white minority government (of which the United States has been a major economic partner) while establishing closer diplomatic ties to the increasingly impatient black majority.

Just yesterday, United Press International reported that the president has been soliciting Republican legislators for their ideas on a policy shift and that Secretary of State George Shultz will announce policy changes when he appears before the Senate Foreign Relations Committee next week.

The Brown appointment -- still far from a certainty, even though his name reportedly has been floated in Pretoria -- could, by one reading, be a part of a grand diplomatic scheme calculated to serve notice on South Africa that it really must move quickly to end apartheid or else risk losing the support of its most important trading partner.

There is another, less hopeful, reading. President Reagan, who has emphasized once again his opposition to tough sanctions, is under increasing domestic pressure to do something. His policy of "constructive engagement" has been interpreted by South Africa not as an enticement to do better but as a wink of support for doing as little as possible. The House of Representatives, to its own surprise, has voted sweeping sanctions against the white minority regime. And yesterday, Sens. Edward Kennedy and Lowell Weicker called for Senate passage of the House bill by Aug. 15 to shock South Africa into understanding that it is at "the very end of the possibility of achieving peaceful change."

The president, similarly caught out last year, when congressional passage of a milder bill of sanctions seemed inevitable, responded by issuing an executive order that, while full of tough talk, had the effect of undercutting the legislative initiative.

The unanswered question now is whether Reagan is climbing aboard the bandwagon of indignation against the South African government or merely looking for a way to slow its momentum.

If Pretoria's intransigence really has exhausted the president's patience to the point that he is willing to institute a tougher policy, the safe bet is that he will be joined by Great Britain's Margaret Thatcher and West Germany's Helmut Kohl, the principal European supporters of the South African government.

Such a development would vastly increase the chances that State President P. W. Botha would take the steps necessary (releasing Nelson Mandela and other political prisoners, unbanning the African National Congress and allowing its exiled leaders to come home) to begin full-fledged negotiations with the black majority.

But if Reagan is only fooling around, or if the Pretoria thinks that he is, it could spell irreversible disaster for South Africans of all hues.

That is the context in which the prospective appointment of a black ambassador must be viewed. Would the expected appointment signal real change? Or would Brown merely be a new deliveryman for the same discredited message of constructive engagement?