FOR ALMOST 50 years, the Federal Bureau of prisons held tenaciously to a rule barring reporters from interviewing individual prisoners. It argued that such interviews would disrupt prison routine and make discipline more difficult. Now almost three years after winning a major court case in which it persuaded the Supreme Court the rule was constitutional, the prison bureau has changed its policy. Director Norman A. Carlson announced the other day that reporters can interview prison inmates, with a few qualifications, if they are willing to talk.

We readily concede that we have a direct interest in this shift of policy. This newspaper challaged the old rule and argued against it unsuccessfully before the Court. We were able ,however, to persuade four justices that the rule violated the First Amendament and the dissenting opinion of one of them, Lewis F. Powell, has been followed rather closely by the Bureau of Prisons in framing its new rule.

This is, it seems to us, a refreshing example of how of bureaucracy can occsionally change its mind even when it doesn't have to Mr. Carlson experimented with news media interviews in some of the minimum security institutions and found they did not create the difficulties some of his predecessors feared. Indeed, he thinks, as we do, that these interviews may turn out to be a net plus for the prison system since they will eventually help the general public understand better what the system's problems and programs are.

We would have been happier, of course, if the government had conceded that these interviews are a matter of constitutional right rather than administration grace. As it is, what the Bureau of Prisons is permitting under Mr. Carlson it can forbid under a future director. And some of the restrictions that Mr. Carlson has placed on those interviews - for example, the right of the bureau to bar subsequent interviews if the reporter fail to verify allegations made during the original one - raise a First Amendment question just as the old rule did. Nevertheless, Mr. Carlson's action does a lot to ease some of the tention between prison officials, inmates and the press.