Old habits die hard. So it's difficult to hope that this week's decision by the military government of Chile to replace 15 years of repression with a measure of freedom will last long.

Not that the change isn't welcome. It means, at least ostensibly, an end to government authority to arrest people for up to 20 days without due process of law, restrict the right of assembly, curb press freedoms, and exile opponents without the right of appeal.But the timing of the move is suspect, coming just six weeks before a plebiscite on continued military rule and only a few days before the ruling junta is to choose the only presidential candidate to appear on the ballot. If voters approve that candidate - probably incumbent president and commander in chief Augusto Pinochet - he will start an eight-year term next March. If not, an open election is to be held within a year, with Pinochet remaining in power until then.

Besides its expedient timing, the decision to lift repressive old emergency powers looks suspect because it comes in response to pressure from various sources, including the political opposition in Chile, the Catholic Church, and the Reagan administration. Basic changes made under duress seldom last long.

Moreover, if the junta is serious about changing its ways, why does it persist in pursuing the same harsh practices right up until virtually the last moment before the change is supposed to take place?

Only a day before the repressive emergency powers were to be lifted, police arrested an opposition editor by breaking into his home. The charges against him? Insulting the armed forces with an article denouncing abuses by the secret police.

Some insult. Such abuses are well-known. Amnesty International, the London-based human rights group, reports that Chile's security forces are linked to 128 assaults or kidnappings in the past 18 months. The victims have ranged from political, union, and community leaders to clergymen, lawyers, and journalists. Their treatment has included kicking, beating, burning with hot metal or acid, and cutting marks into their flesh.

Human nature being what it is, a regime that resorts to such tactics can be expected to look for a pretext to rescind its decision to abandon them. That being the case, those who worked so long and hard to get Chile to cancel its repressive emergency powers had better work just as diligently to make sure the new policy sticks.