FOR MORE THAN 41/2 years President Reagan practiced "constructive engagement" with South Africa, and reform there proceeded at a pace that those who wish to end apartheid found completely unacceptable. Two days after he initiated mild sanctions, the Pretoria government pledged to restore South African citizenship to the millions of blacks it had previously stolen that birthright from, and the next day a weather-vane advisory commission called for abolition of the pass laws.

It cannot all be due, of course, to Mr. Reagan's new receptivity to sanctions. The South Africans, insisting that they did not merely bend to pressure, say the changes were in the works for a long time. Yet less than a month ago President P. W. Botha apparently cranked up to make such changes, and didn't. Soon new blows were delivered to the South African economy by private banks worried about their money, and then Congress pressed Mr. Reagan into reversing course on sanctions. Did Pretoria conclude it had best make a gesture to him in order to earn passage back toward his patronage?

The reforms now being cited hardly prove a South African change of heart. Restoration of citizenship undermines the odious intent of apartheid to spin off blacks to tribal "homelands" and thus make them foreigners in their own country. The restoration heightens the pressure on Pretoria to permit blacks an acceptable political role in South Africa. The pass laws, a cornerstone of apartheid, control where blacks may live and work. Abolition would remove a savage instrument of white repression.

The ruling whites offer up these measures as major alterations in apartheid. But they come so late and begrudgingly that a great many blacks are likely to find them thin gruel. They are what Bishop Tutu -- who, keep in mind, falls in the category of racial moderate among blacks -- calls "piecemeal reform" of a sort that "no longer excites us. . . . I don't want apartheid reformed, I want it dismantled." By dismantling, blacks mean ending the system that denies them full political rights. In all the twisting and turning of the Botha government, no signs are yet visible that it understands the rightness, urgency and inevitability of that goal.

Some whites in South Africa, to be sure, may be thinking of taking this step. Five prominent businessmen, including an Afrikaner, are even today in Zambia for a first acknowledged meeting with the outlawed African National Congress. The five are at once defying the government's stated wishes and conducting a political reconnaissance. This is the sort of initiative the government is going to have to take. As long as it not only disenfranchises blacks but also locks up their natural leaders -- Allan Boesak is the latest of note to be jailed -- it ensures that blacks will turn to other means of struggle, and it renders suspect and marginal its other initiatives.