Robert Bourassa seems to have undergone something of a change of heart. In the first, euphoric days after the Meech Lake Accord was signed, the Quebec premier maintained the accord's ''distinct society'' clause, together with the responsibility of the Quebec government to ''preserve and promote'' the province's distinct identity, had changed everything.

''The whole Constitution,'' the premier bragged to the National Assembly, ''including the charter, will be interpreted and applied in the light of the section on our distinct identity. This . . . will enable us to consolidate what has already been achieved and to gain new ground.''

More recently, aware of the unease over Meech Lake in the rest of Canada, Bourassa has taken up the line to which his federal allies have held fast all these months - that the clause is merely an affirmation of sociological fact - and given it a novel twist. The clause would change nothing, Bourassa now plaintively insists, because the courts already take Quebec's distinctiveness into account through the charter's ''such reasonable limits'' clause.

LEGAL HAVOC

So they do. In that case, why include it? Or why not simply put it in the preamble, the very demand on which the Bourassa government was elected in 1985? Because - and this is the twist - not to do so would change everything. If the clause were not included, and given interpretive weight, Bourassa says, that would be a ''step backward'' from the status quo, before Meech Lake had been invented.

Understand? Adding a clause to the Constitution changes nothing. But not adding the clause changes everything. This is worrisome, since there are any number of changes that have not been made to the Constitution. Who knows what legal havoc they may have caused?

But enough carping. Those who maintain the clause does mean something, and not nothing, have a duty to spell out what, specifically, it might mean. Let us take three contemporary issues in Quebec, and see how it might be applied.

Quebec's birth rate, having sunk to the lowest level in the developed world, has lately begun to rebound. No one can be sure just why this is so, but the government feels its program offering a cash bonus for each new child in a family has had some effect. The problem is that most of the increase in births has come not from the French-speaking population, but from allophone immigrant families. Could that not justify - and protect from a charter challenge - a policy of limiting the bonus only to families with French-speaking parents?

The Montreal Catholic School Commission caused a fuss by its plan to survey parents on whether they would prefer separate schools, according to whether children were of ''pure'' Quebecois or immigrant parents. But that's not to suggest such a plan might not be popular. One of the side effects of the law restricting access to English schools to the children of parents educated in English in Canada has been to forcefeed allophone immigrants into the French school system. In some districts, French is the minority language on the schoolyards. To guard against the assimilation of francophone pupils, could that not justify to the courts the necessity of a segregated school system?

The Canadian Radio-television & Telecommunications Commission recently heard testimony in favor of raising the French-content quotas on Quebec radio. This brought protests from the owners of francophone stations that they would lose listeners to the English stations. All right, said some: raise the French quotas for English stations, now set at 5% as well - say, to 35%.

The CRTC, however, is unlikely to raise quotas to levels that would satisfy Quebec nationalists. And communications, the courts have ruled until now, are a federal jurisdiction. But could not Quebec assume some powers over communications, say in matters concerning the French language, in order to ''preserve and promote'' its distinct identity? Ottawa would not have lost its jurisdiction. It would simply be sharing it, as it does in matters of immigration.

This is not to suggest these sorts of things are probable. Nor is Quebec any more prone to violations of individual rights than any other province. But it would be the only province in which such behavior was legal.